BEIRUT, Lebanon — Thousands of Syrians took to the streets on Friday in demonstrations that seemed to be dwindling in size, while clashes continued between security forces and army defectors in central Syria, residents and witnesses said.

Human rights activists said that at least 13 people were killed when troops opened fire at their protests. Seven of them were shot in Homs; two in the suburbs of Hama; three in the suburbs of the capital, Damascus; and one in the city itself.

Also on Friday, the American ambassador to Syria, Robert S. Ford, said that an attack on his convoy by government supporters on Thursday was more violent than had been reported. He said one man jumped on the hood of one vehicle and tried to kick in the windshield while another clung to the vehicle’s side, trying to smash a window. Others threw concrete blocks and attacked the vehicles with metal rods.

The attack on the American diplomat in the capital followed a week of clashes between army defectors and troops loyal to President Bashar al-Assad.