The video, broadcast last week by the Telshan television station, shows B. Sivakumar being hit with a baton by a man identified as a policeman. Sivakumar ran into the sea to escape the beating but the policeman followed and continued to strike him as he clasped his hands begging for mercy. He later drowned.

Sivakumar's family told the TV station that he was mentally ill.

Police spokesman Nimal Mediwaka said two policemen were arrested in connection with the death. "They are wrong. That's why we have taken them into custody," he said.

About 200 protesters gathered in Colombo and accused police of killing suspects in custody. "Protectors or thugs?" they shouted. A leaflet distributed at the demonstration said 23 suspects have died mysteriously this year in police custody.

Mediwaka denied the allegation that many people had died while in custody, saying some had been killed trying to escape. He did not provide any specific figures.

Mano Ganeshan, an opposition lawmaker who organized the protest, said Sivakumar had been caught by the police officer throwing stones at passing vehicles.

"If someone stones vehicles, it is understood that he is a mentally ill man. If a policeman can beat him and kill him in broad daylight, it proves the collapse of law and order in this country," Ganeshan said.

Sri Lanka has been widely criticized over its human rights record, particularly during its decades-long civil war with Tamil Tiger rebels that ended earlier this year.

A U.S. State Department report last month alleged that government forces carried out attacks on civilians and hospitals and said the actions could amount to war crimes.

A European Union report last month said Sri Lanka has failed to respect human rights _ a finding that could threaten its trading rights with the EU.

Also Wednesday, police fired tear gas and a water cannon to disperse a protest by unemployed demonstrators demanding jobs who tried to enter a high-security area in Colombo that houses key government and military offices.