Donald Trump was sworn in as president of the United States on January 20, pledging to put "America first" at all times but also to strengthen existing alliances, forge new ones, and let the country "shine as an example for everyone to follow."
In a brief inaugural address after taking the oath of office while thousands protested against him on the streets of Washington, some violently, Trump promised a "great national effort to rebuild our country" for all its people.
But he lashed out at opponents and the political establishment by charging that the elite in Washington has prospered at the expense of ordinary citizens - whom he called victims of "American carnage" -- and vowed to hand power that he said is now held in the capital "back to you, the American people."
Trump, 70, defeated Democratic rival Hillary Clinton in the November 8 election after a contentious campaign clouded by what U.S. intelligence agencies say was Russian interference in his favor as well as by concerns among critics about his blunt remarks and his statements on a range of issues including immigration, Islam, and climate change.
LIVE BLOG: Donald Trump's Inauguration
At home, he pledged to "protect our borders from the ravages of other countries making our products, stealing our companies, and destroying our jobs."
"From this day forward it's going to be only America first," Trump said, repeating: "America first."
"America will start winning again, like never before," he said.
WATCH: Trump Vows To Put 'America First'
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Trump Vows To Put 'America First'
Abroad, Trump said, "We will reinforce old alliances and form new ones -- and unite the civilized world against radical Islamic terrorism, which we will eradicate completely from the face of the Earth."
The United States will not "seek to impose our way of life on anyone, but rather to let it shine as an example for everyone to follow," he said.
Trump -- the oldest person ever to start an initial term as U.S. president -- was sworn in after attending a morning church service and a meeting with outgoing President Barack Obama at the White House along with their wives, Michelle Obama and Melania Trump.
"Mr. President-elect, how are you?" asked Obama, who had left the Oval Office for the last time in eight years as president minutes earlier and said "Thank you" to the American people.
In his address, Trump thanked the Obamas for "their gracious aid" throughout the period of transition of power.
WATCH: Obamas Welcome Trumps To White House
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Obamas Welcome Trumps To White House
After a luncheon with lawmakers and dignitaries at the Capitol, Trump and the new first lady rode a limousine that inched to the White House in the midst of a ceremonial parade, red and blue lights flashing as security agents walked slowly beside the vehicle and onlookers watched from the sidewalks.
Defeated candidate Clinton received a cheer from the crowd when she arrived at the ceremony. Members of the U.S. Congress also attended, but more than 60 of the Democrats in the U.S. House of Representatives boycotted the event.
Tight Security, Protests
Security was tight with thousands of law-enforcement officers deployed and kilometers of crowd barriers set up to handle about 900,000 people who had been expected to turn out in the capital.
The crowd of spectators on the National Mall was smaller than when Obama started his first term in 2009, according to aerial footage.
Rifts revealed and aggravated by the election campaign were also on plain display, with thousands of people protesting, clashes breaking out between police and demonstrators, and dozens of arrests.
Most of the protests -- including those by a range of anti-racist, feminist, pro-immigration, antiwar, and marijuana legalization groups concerned about Trump's views -- were peaceful. Some marchers chanted: "No deportation, no KKK, no fascist USA!"
But about 90 minutes before the swearing-in ceremony, activists smashed store and car windows and clashed with police in riot gear who responded with chemical spray and stun grenades.
About 500 people, some wearing masks, marched through the city's downtown in the late morning, using hammers to claw up pieces of pavement and smashing windows at a Bank of America branch and a McDonald's restaurant.
WATCH: Inauguration Protest In Washington Turns Violent
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Inauguration Protest In Washington Turns Violent
After the ceremony, an RFE/RL correspondent said, police clashed with protesters near the parade route a few blocks from the National Mall who tore up cobblestones and bricks, overturned newspaper vending boxes and tossing them into the streets, and shouted obscenities targeting Trump and police.
The sound of stun grenades rang out repeatedly and chemical spray billowed in the air. Helmeted police holding shields and wielding batons cleared some 50 protesters from an intersection, shouting "Move, move, move!"
Washington authorities said they arrested at least 217 people for violence during protests against Trump.
Those arrested were charged with vandalism and violence against law-enforcement officers, six of whom suffered minor injuries during the clashes, Washington's mayor said after the inaugural festivities ended on January 20.
Trump, a real estate developer and former reality TV star with no previous government experience, faces a tough task if he seeks to bridge divisions.
He lagged behind Clinton by nearly 3 million ballots in the popular vote, and a Washington Post-ABC News survey conducted earlier this week found his approval rating was the lowest of any incoming president-elect since Jimmy Carter in 1977.
"The majority of Americans did not vote for this man, and probably support... agencies doing the work of protecting the consumer and the environment,' said Maryann Daly, a 66-year-old retiree from Washington who held a sign reading 'Honor the Paris Climate Agreement'."
'Too Much Focus Outside U.S.'
Anti-Trump protester Ken Herringer, a 54-year-old tax preparer from Silver Spring, Maryland, told RFE/RL: "I do believe in freedom, and I have a feeling this new administration might try to roll back some of the gains we've made over the past eight years."
But Trump supporters who came to Washington spoke enthusiastically about the prospects for the United States and its people during his presidency.
"We're going to make America great again, because for the last, really,16 years we've had too much focus outside of the United States," Meg Kilgannon, a 44-year-old teacher of natural childbirth techniques from Centerville, Virginia, told RFE/RL.
"We need to focus on our own country, get our own house in order, and do things for the American people," she said.
Jon-Paul Oldham, a firefighter from the state of Connecticut, told the Associated Press that he was "hoping for some unity" and suggested all Americans should rally behind Trump.
"Wanting him to fail is like wanting the plane to crash, but you're on the plane," Oldham said.
WATCH: Profile -- America's New President, Donald Trump
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Profile: America's New President, Donald Trump
One of the largest demonstrations in Washington was organized by the ANSWER Coalition, an umbrella group consisting of antiwar and civil rights organizations.
"It's Day One, we're saying, of a larger era of resistance, and we believe we're going to send a very powerful message to Trump and the government," said Ben Becker, an organizer with the group. "The Trump agenda is very comprehensive. It includes attacks on Muslims, immigrants, on women's rights, on workers' rights."
Rolling Back Obama Policies
Confirmation hearings for Trump's cabinet nominees this week have been marked by often testy questions from lawmakers, particularly Democrats, amid concerns about the nominees' qualifications and potential conflicts of interest.
Earlier this month, U.S. intelligence agencies said they had concluded "with high confidence" that Russian President Vladimir Putin "ordered an influence campaign in 2016 aimed at the U.S. presidential election," with the goals of undermining faith in U.S. democracy, denigrating Clinton, and harming her electability. They charged that Putin and the Russian government "developed a clear preference" for Trump over Clinton in the election.
While speaking of unity, Trump was readying several measures to roll back Obama's policies through a series of executive orders.
Minutes after Trump was sworn in, a statement posted on the White House website said that defeating Islamic State "and other radical Islamic terror groups" would be the highest priority of an "America first" foreign policy, which it said would be centered on "peace through strength."
The statement added that the United States would "pursue aggressive joint and coalition military operations when necessary" to achieve that goal.
In addition, the statement promised to "rebuild the American military," reversing what it said had been a substantial downsizing of the army and navy since 1991 -- the year the Soviet Union collapsed.
It also included a vow to "embrace diplomacy," saying: "The world must know that we do not go abroad in search of enemies, that we are always happy when old enemies become friends, and when old friends become allies."
A separate statement focusing on the military said the United States "cannot allow other nations to surpass our military capability" and "will pursue the highest level of military readiness."
It said the United States would "develop a state-of-the-art missile defense system to protect against missile-based attacks from states like Iran and North Korea."
It did not say whether the system would differ from those in place or under development, including a European missile shield that Russia has vociferously opposed.
The foreign policy statement said the United States will seek to bolster its economy by "fighting for fair but tough trade deals" and will start by withdrawing from the Trans-Pacific Partnership, which Obama negotiated.
Another statement said that Trump was "committed to building a border wall to stop illegal immigration," an apparent reference to a wall on the border with Mexico that was a key campaign promise and target of criticism from opponents.
Ahead of Inauguration Day, aides said some of Trump's first actions would include a freeze on U.S. government hiring, suspension of climate change regulations, suspension of immigration from countries associated with Islamic extremism, lifting of restrictions on coal mining and oil drilling, and halt to federal funding for U.S. "sanctuary cities," which refuse to arrest immigrants living in the country illegally.
Congratulations and exhortations came in from world leaders throughout the day, with Russian Prime Minister Dmitry Medvedev and Foreign Minister Sergei Lavrov both expressing optimism and saying Russia is ready to do its share to improve relations with Washington.
Trump has said he wants to improve U.S.-Russia relations, which have sunk to lows unseen since the Cold War amid rancor over Moscow's seizure of Ukraine’s Crimean Peninsula in 2014, its backing of separatists in a war that has killed more than 9,750 people in eastern Ukraine, its involvement in the Syrian conflict, and other issues.
Trump's praise of Putin and statements about Russia ties, as well words in support of key ally Britain's exit from the European Union and appraisal of NATO as "obsolete," have caused jitters in Europe about his intentions and sparked concerns that he could scrap sanctions imposed on Moscow over its interference in Ukraine and step back from U.S. commitments abroad.
In an interviewed published on January 20 in the Financial Times, British Prime Minister Theresa May said that she thinks Trump "recognizes the importance and significance of NATO," and that she is "confident" he will "recognize the importance of the cooperation we have in Europe to ensure our collective defense and collective security."
NATO Secretary-General Jens Stoltenberg congratulated Trump and said NATO's "strength is as good for the United States as it is for Europe."
Indian Prime Minister Narendra Modi, Ukrainian President Petro Poroshenko, and Kosovar President Hashim Thaci tweeted their congratulations and hope for cooperation with Trump's administration.
Pope Francis urged Trump to use ethical values as his guide and take care of the poor and the outcast during his presidency.
"I pray that your decisions will be guided by the rich spiritual and ethical values that have shaped the history of the American people and your nation's commitment to the advancement of human dignity and freedom worldwide," wrote Francis.