BEIRUT, Lebanon — Dozens of pro-government Syrians attempted to assault an American diplomatic delegation that included the ambassador on Thursday, striking its motorcade as it traveled to a meeting with an opposition figure in Damascus and then trying to break into an office where the meeting was held, essentially trapping the participants inside for 90 minutes.

The United States protested the episode and suggested that the attackers had been deliberately allowed to harass the diplomatic delegation by Syrian security forces, who arrived belatedly to provide safe passage for the Americans to leave. The ambassador, Robert S. Ford, an outspoken critic of President Bashar al-Assad of Syria, was reported safe, but some vehicles in his motorcade were damaged.

“We condemn this unwarranted attack in the strongest possible terms,” Secretary of State Hillary Rodham Clinton said in a statement. “Ambassador Ford and his aides were conducting normal embassy business, and this attempt to intimidate our diplomats through violence is wholly unjustified.”

It was the second attack on American diplomats in Syria since the antigovernment uprising started in mid-March. In July, after Mr. Ford’s visit to Hama, a restive city in central Syria, supporters of the government attacked the embassy’s compound.