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.
Mr Ford remains in the building, which is still surrounded, two hours after his arrival as he waits for security forces to escort him out.
Syrian media has previously accused Mr Ford of inciting protest.
He angered Damascus 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.
Mr Abdul Azim, who heads the outlawed Arab Socialist Democratic Union party, said the ambassador's arrival at his office 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.
9/29/11
US ambassador Robert Ford pelted with tomatoes in Syria
6:29 PM
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