The violence on January 19 came after tens of thousands of protesters gathered in Independence Square in defiance of new antiprotest legislation.
Protesters attacked police with sticks as they tried to push their way toward the parliament building, which had been cordoned off by rows of police and buses.
Stun grenades were used and smoke was seen above the crowd.
One police bus was destroyed and set alight. The flames of the burning bus could be seen from far away.
As clashes continued reports emerged that boxer-turned-politician Vitali Klitschko had met with President Viktor Yanukovych at his presidential residence outside Kyiv.
Klitschko later tweeted that Yanukovych had agreed to set up a committee to settle the political crisis.
On his website, Yanukovych said that he had tasked a working group -- headed by national security head Andriy Klyuev -- to meet with opposition representatives on January 20 to work out a solution to the crisis.
The White House urged an end to the violence, with National Security Council spokeswoman Caitlin Hayden saying that Washington was deeply concerned and urging "all sides to immediately de-escalate the situation."
Hayden said Ukraine's government "has moved to weaken the foundations of Ukraine's democracy by criminalizing peaceful protest and stripping civil society and political opponents of key democratic protections under the law."
She called on Ukraine to repeal recent legislation limiting protests, to remove riot police from downtown Kyiv, and to start talking to the opposition.
"The U.S. will continue to consider additional steps -- including sanctions -- in response to the use of violence," Hayden said in a statement.
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The new legislation -- signed into law by Yanukovych on January 16 -- bans any unauthorized installation of tents, stages, or amplifiers. It also prohibits protesters from covering their faces or from wearing construction hats.
Many demonstrators wore medical, ski, and carnival masks in defiance of the new regulations and others had kitchen pots and colanders on their heads. Many others wore construction hats.
The legislation allows for prison terms of up to 15 years for the "mass violation" of public order.
The new laws also require nongovernmental organizations to register as "foreign agents" if they are funded from abroad, mirroring a similar rule on the books in Russia.
The new legislation was passed by a show of hands in the parliament after the opposition blocked access to the podium and the electronic voting system.
SLIDESHOW: Kyiv protests turn violent
Opposition lawmakers said the way the laws were passed was unconstitutional and declared the legislation null and void.
Also on January 16, opposition leader Arseniy Yatsenyuk declared the start of opposition-sponsored referendums and elections for new government bodies and officials in Ukraine.
"We are starting popular voting on the lack of confidence [in the government] and over the dismissal of [Ukrainian President] Viktor Yanukovych," he said.
Yatsenyuk added that ballots would be held on a "declaration of support for and confirmation of the authority of the Ukrainian People's Council" as well as on "the creation of a new constitution for Ukraine."
He also said a popular vote would be held to elect an alternative Kyiv City Council and mayor of Kyiv.
Tension Mounting
Tensions remain high in Ukraine since Yanukovych's abrupt decision late in November not to sign a deal with the EU, sparking some of the biggest protests in the country since the 2004 Orange Revolution.
Yanukovych allegedly changed course after pressure from the Kremlin, which has since offered financial aid and cheaper gas to cash-strapped Ukraine.
Analysts say the protests do not appear to have shaken Yanukovych's hold on power. But Yanukovych's decision late on January 17 to sack his chief of staff, analysts say, indicates that tensions are simmering within the Ukrainian leader's inner circle.
The president's office gave no reason for the dismissal of Serhiy Lyovochkin.
Lyovochkin was reported to have considered stepping step down in November after riot police brutally dispersed student protesters.
The AFP news agency quotes an unnamed Ukrainian official as saying Yanukovych's spokeswoman, Darka Chepak, is also considering stepping down.
"Turmoil in regime in Kyiv. Reports of resignations, dismissals and general uncertainty," Swedish Foreign Minister Carl Bildt wrote on Twitter.
The United States and European Union have called the new legislation passed on January 16 antidemocratic. Ukraine's opposition accuses Yanukovych of trying to install a "dictatorship."
