All but two of those killed in the attack were policemen who were guarding Islamists marking the first anniversary of an army commando raid on Islamabad's Red Mosque.

The attack will raise questions about the new government's policy of trying to end militant violence through negotiations, and increase concern about prospects for the country, a nuclear-armed United States ally making a transition to civilian rule.

The government is led by the Pakistan People's Party (PPP) of former prime minister Benazir Bhutto, who was killed in a suicide attack in Rawalpindi last Dec 27.

Ms Bhutto's widower Zardari, who took over as leader of the party after her assassination, said Sunday's attack was despicable and must be condemned by all.

'The Pakistan People's Party realises the grave threat that such terrorist activities pose to the country, and the PPP government will do everything possible to check the activities of such elements,' he said.

Pakistani investigators yesterday found a severed head at the scene and were examining it, although city police chief Asghar Raza Gardezi said it was too early to draw firm conclusions. No suspects had been detained so far.

No one has claimed responsibility for the blast, although police had earlier pointed to a teenage boy as the suspected attacker.

'All witnesses say that a 15- or 16-year-old boy, who had a light beard and wore a white shalwar kameez... came walking towards our police and blasted himself,' said Interior Ministry chief Rehman Malik.

The explosion happened several hundred metres from the Red Mosque shortly after a tightly guarded Islamist rally there had ended.

Several thousand Islamists, including members of banned groups, had listened to fiery speeches to mark the first anniversary of the army raid on the complex.

More than 100 people died when commandos stormed the Red Mosque on July 10 last year after a week-long siege that began when gunmen from the mosque clashed with police.

A few weeks after the siege, 13 people, mostly policemen, were killed in a suicide bombing similar to Sunday's attack.

President Pervez Musharraf, whose power has withered since his allies were defeated in a February election and who has been facing calls to step down, said last Friday that more radical mosques would emerge if extremism was not tackled.

Meanwhile yesterday, a string of five 'low-intensity explosive' blasts wounded at least 25 people in Karachi, police said.

The blasts came within about an hour of each other, and TV footage showed ambulances rushing people through the southern city's crowded streets to hospitals.

Both residential and commercial areas were struck, and some sources said at least one person was killed. But it was unclear if any security officials were among the wounded or what the motivation behind the attacks was.

REUTERS, ASSOCIATED PRESS