Over 600 police personnel and troops took part in the joint operation, which started before dawn and lasted for five hours, in the Maligawatta area near Colombo's city centre.

The government, which less than a week earlier announced presidential elections likely to be held by late January, has ordered police and the army to crack down on criminals.

Routine searches are often seen in the capital for suspected Tamil separatist rebels, but this was the first time in recent years police sought military cooperation in a cordon against criminals.

Senior Deputy Inspector General H M G B Kotakadeniya has warned that police will not hesitate to make precautionary arrests of known underworld figures who pose a threat to the conduct of the presidential elections.

Both the ruling People's Alliance (PA) and the mainopposition United National Party (UNP) have set off for a hectic campaign for the crucial election to elect a president for the next six years.

Political parties in Sri Lanka have in the past been known to seek the support of gangs, both to assist their campaign work and to intimidate supporters of rival parties.

President Chandrika Kumaratunga is running for re-election as the PA candidate, while opposition leader Ranil Wickramasinghe, a former prime minister, is contesting the poll as the UNP's candidate.

The Marxist JVP (People's Liberation Front) have fielded Nandana Gunatillake as their candidate. At least four more candidates are to run for other political parties or as independents.

The presidential election is seen as a forerunner to parliamentary polls likely to be held before the middle of next year.