Supporters of the main opposition United National Party and a group of ruling party dissidents led by former foreign minister Mangala Samaraweera marched from Campbell Park in Colombo to the nearby Hyde Park for a rally.
Anti-government activists poured into the streets despite warnings by the government that Tamil Tiger rebels had infiltrated the city of 650,000 people with truck and car bombs.
Fewer vehicles were on the streets Thursday as many people remained indoors fearing trouble during the opposition rally, police said, adding that they stepped up their presence.
"We have placed anti-riot squads on stand-by," a police official at the rally said.
The activists were denouncing the government over its human rights record and displayed a white van with photographs of hundreds of people who had "disappeared" in the past year.
International rights groups have said that about 1,000 people have been murdered or disappeared in the past year amid an escalation of fighting betwe