"Bullets started flying over our heads like heavy rain," a witness was quoted as saying by the Associated Press news agency.
A witness in Douma told Reuters he had helped carry three people with bullet wounds to their legs.
One resident in Homs, a city of 700,000 people in the west, told the BBC she had heard shooting and believed three separate protests were under way in the city.
"The security forces are just dispersing the protesters using live bullets," said the resident, who did not wish to be named.
In Hama, a city in central Syria similar in size to Homs, security forces are said to have also opened fire on a crowd of protesters.More >>