Supporters of Syrian President Bashar al-Assad have pelted US ambassador Robert Ford with tomatoes as he met an opposition figure in Damascus.
Veteran politician Hassan Abdul Azim said about 100 protesters tried to get into his office as Mr Ford arrived and then surrounded it, trapping him.
Mr Ford, who has been accused of inciting protests, was still awaiting a police escort two hours later.
Meanwhile, Syria has accused the US of inciting violence against its military.
"Recent statements from American administration officials... clearly indicate that the United States is involved in encouraging armed groups to practice violence against the Syrian Arab Army," a foreign ministry statement said.
It is thought the statement was referring to comments made by US state department spokesman Mark Toner, who said on Tuesday that it was "not surprising" that the opposition were using violence against the military.
Mr Ford has angered Damascus in the past, notably by visiting the central city of Hama with his French counterpart in July.
It led to both the French and US embassies coming under attack from supporters of the Assad regime.
International pressureMr Abdul Azim, who heads the outlawed Arab Socialist Democratic Union party, said the ambassador's arrival at his office on Thursday led to a demonstration.
"As soon as the ambassador came in at around 11:00 (08:00 GMT), we heard a noise outside and hostile slogans being chanted. The demonstrators tried to attack the office," he told AFP news agency.
Syria is under international pressure to stop using force to suppress protests that began six months ago.
The UN estimates that more than 2,700 people have been killed across Syria since the crackdown began.
And the UN Security Council is set to vote on a Europe-led resolution which demand an "immediate end to all violence" by the state, and threatens more sanctions against Mr Assad and his inner circle if they do not comply.
The EU and US both already have sanctions in place.
