The military claims it has been taking extraordinary measures to pre-empt attacks in Colombo.

But the latest devastation, caused by the most powerful blast in the city to date, has added a serious new dimension to the safety of the capital's citizens.

'Not only have the rebels struck in the very heartland of the administration, but they have had the temerity to engage the numerically superior armed forces in gunbattles,' a leading military analyst said yesterday.

In August, city police had come under heavy criticism following an explosion triggered by a suicide bomber who had been arrested and released 36 hours before.

Analysts believe several recent changes at the top of the police force have been prompted by the August blast, which left 24 dead and 150 injured.

Intelligence reports following that explosion had indicated elite teams of Black Tigers, the Tamil Tigers' suicide unit, had infiltrated the city and were planning a bombing spree.

Wednesday's huge explosion, from which black smoke billowed, obliterating the city skyline for kilometres, caused thousands of passersby to flee for their lives.

Security forces have over the past six months been mounting full-scale hunts for guerillas in the city.

The operations had been prompted by the capture of several suspected members of Tamil Tiger suicide squads, trying to slip past the city's security cordons.

Several of them are believed to be explosives experts.

But security forces have perhaps been concentrating more on protecting their own threatened headquarters, the military top brass and politicians, rather than key utility areas.