In Washington, Undersecretary of State Nicholas Burns denounced the assassination as an "act of terrorism."
British Foreign Secretary Margaret Beckett said her government was "dismayed" at the assassination, saying "there are enough problems in Lebanon already."
Israeli Foreign Minister Tsipi Livni, speaking in London at a joint news conference with Beckett, said the killing "is another example of the kind of region we're living in."
Murder Comes Amid Political Crisis
Gemayel, who was Lebanon's industry minister, was gunned down after the convoy that was driving him through the Christian Sin el-Fil neighborhood was rammed.
Gemayel died en route to hospital.
Lebanon, still recovering from last summer's devastating war with Israel, was already facing a deep political crisis following the recent defection from the government of six pro-Syrian ministers.
Gemayel's assassination looks set to make that crisis worse.
Prior to the killing, pro-Syrian Hizballah and its Christian ally Michel Aoun, a former prime minister, said they were preparing street protests to topple Prime Minister Fouad Siniora's government. They accuse of it being allied with the United States and say it has lost its legitimacy since Shi'a Muslims are no longer in the cabinet.
Siniora warned this week that any anti-government protests could turn violent.
The Hand Of Syria?
The assassination recalled the killing of Rafik Hariri, the charismatic former prime minister whose motorcade was destroyed by a massive explosion in February 2005.
Hariri's son Saad, the leader of the anti-Syrian parliamentary majority, was forced to interrupt a televised news conference today when he was handed a note informing him of the attack on Gemayel.
Hariri later implicated Syria, stating, "we believe the hand of Syria is all over the place."
Many Lebanese as well as a United Nations inquiry have implicated Syria in Rafik Hariri's killing.
Gemayel, elected to parliament in 2000 and again in 2005, is the third Lebanese anti-Syrian figure to be assassinated since Hariri's killing in February 2005.
He was also a member of one of the country's leading political families. His party, the Phalange Party, was founded by his grandfather and his father, Amin, is a former president. His uncle, Bashir, was killed in September 1982 after he was elected president during Israel's invasion of Lebanon.
Syria today strongly condemned Gemayel's assassination, as did the Shi'ite militant group Hizballah.
On the streets of Beirut, the killing has triggered scenes of chaos.
Dany Khouri, a freelance Lebanese journalist, told RFE/RL that "the streets have just gone mad."
Khouri says Beirut residents fear that chaos and violence could break out, particularly as Lebanon is due to mark Independence Day, the day that Lebanon broke away from France in 1943.
With Gemayel's death, the resignation or death of two more ministers would bring down Siniora's government.
Despite the resignations of the pro-Syrian ministers, Siniora's cabinet last week approved draft United Nations' statutes to set up a tribunal to try Hariri's killers.
There is now another murder to solve.