Security forces armed with tough anti-terror laws increased road blocks and random checks within the city of 600,000 people and carried out searches against the Liberation Tigers of Tamil Eelam (LTTE), a police official said.

"We have got thousands of constables and troops from the provinces to beef up security in Colombo," a police officer said. "We are maintaining a high state of alert."

Some roads in the capital were also closed as security forces rehearsed for Sunday's independence day commemorations along Colombo's sea-front Galle Face promenade.

Sri Lanka won independence from Britain in 1948. With the ending of 133 years of British rule, ethnic tensions between the majority Sinhalese and minority Tamils erupted into the open and spawned Tamil Tiger rebels in 1972.

More than 60,000 people have since been killed in the Tamil separatist conflict led by the LTTE. Diplomatic attempts to bring the two sides to the negotiating table have ended in failure.

Tiger guerrillas carried out two suicide bombings killing more than 20 people ahead of the 50th Independence Day in 1998.