Pierre Gemayel, shot dead near Beirut, was the fourth outspoken anti-Syrian critic to be killed in Lebanon since the 1995 assassination of former Prime Minister Rafik al-Hariri sparked a period of political upheaval in the country.
US President George W. Bush, speaking in Hawaii, called for a full investigation to "identify those people and those forces behind the killing."
The industry minister's death coincides with sharp differences between Lebanon's anti-Syrian majority and the pro-Damascus opposition led by Hizbullah, which is determined to topple what it sees as a pro-US government.
Finland, which holds the rotating European Union presidency, urged all political factions in Lebanon to "refrain from activities that would further endanger the political stability ... (and) reiterates the EU's full support to the legitimate and democratically elected Lebanese government."
British Prime Minister Tony Blair also voiced support to Siniora, whose government has been rocked by a devastating July-August conflict between Hizbullah and Israel and the resignation of six pro-Syrian ministers this month.
"We need to do everything we can, particularly at this moment, to protect democracy in Lebanon and the premiership of Prime Minister Siniora," Blair said.
SYRIA, IRAN ALSO CONDEMN
Many Lebanese blame Syria for the killing of Hariri in a suicide truck bombing in February 2005.
Damascus denies involvement, although a UN commission investigating the assassination has implicated senior Lebanese and Syrian security officials.
Bush stopped just short of assigning blame to Syria and Iran for the killing of Gemayel.
He said the United States backed Lebanese efforts to "defend their democracy against attempts by Syria, Iran and allies to foment instability and violence in that important country."
Syria and Iran, key backers of Hizbullah, were quick to condemn the killing.
"This is a crime aimed at destabilising Lebanon. Syria is careful about preserving Lebanon's security, unity and civil peace," the official Syrian news agency SANA said.
Iranian Foreign Ministry spokesman Mohammad Ali Hosseini said: "No doubt this act has been carried out by Lebanon's enemies ... that don't want Lebanon to become a symbol ... of the victory of resistance in the region."
Arab leaders called for calm, while steering carefully clear of blaming any particular side for the violence.
"The Lebanese have to unify their ranks at this phase and not allow the preachers of death, disunity and destruction to succeed in creating a split in the cohesion of the Lebanese people," Jordan's King Abdullah said.
Egyptian President Hosni Mubarak said: "We call the Lebanese parties to engage in dialogue for the interest of Lebanon. There should not be any differences, in order not to lose Lebanon."

Copyright Reuters, 2006