Syrian government supporters attacked the US Embassy in Damascus in protest over the ambassador's recent visit to an opposition stronghold.
They smashed windows, raised a Syrian flag and scrawled graffiti calling the American ambassador a "dog".
French Embassy security guards in the capital fired in the air to hold back supporters of President Bashar Assad's regime who were also protesting at the French ambassador's visit to the same city, Hama, in central Syria.
Protesters smashed embassy windows and shattered the windshield of a diplomatic SUV outside the compound. The French flag was removed and replaced with a Syrian one.
"God, Syria and Bashar. The nation that gave birth to Bashar Assad will not kneel," read graffiti written outside the embassy. One witness said three protesters were injured when guards beat them with clubs. The witness asked not to be identified because of the sensitivity of the situation.
A US official said the Obama administration will formally protest over the attack and may seek compensation for damage caused when a mob breached the wall of the compound before being dispersed by Marine guards.
The Syrian regime called the French and American ambassadors' visits to Hama last week interference in the country's internal affairs and accused the envoys of undermining Syria's stability.
The protests erupted after US Ambassador Robert Ford harshly criticised the Syrian government's crackdown on a popular uprising.
Some 1,600 civilians and 350 members of security forces have been killed since demonstrations began, activists say. Syria blames what it calls "armed gangs" and Muslim extremists for the violence.
Hiam al-Hassan, a witness, said about 300 people had gathered outside the French Embassy while hundreds others were at the American diplomatic compound.