ISLAMABAD, PAKISTAN - A powerful suicide explosion killed at least 15 people and injured dozens of others in Islamabad on Sunday evening, shortly after a large protest rally marking the one-year anniversary of government forces' raid on a radical mosque. Most of the dead were police officers.

The blast, which appeared to have targeted security forces, poses a sharp new challenge to Pakistan's coalition government, which has been struggling in its efforts to formulate a policy for dealing with Islamic militants.

The explosion occurred just before 8 p.m. at a police post close to a popular market and only a few blocks from the Red Mosque.

The capital had been tense in anticipation of the anniversary. Preachers at the Red Mosque have employed fiery anti-government rhetoric in recent days. The mosque's supporters, including female students in all-enveloping black burqas, staged rallies adjacent to the site for the past several weeks.

Much of the fury was directed at Pervez Musharraf, who, although still the country's president, wields much less authority now that he has given up his military post and his political opponents control the government.

The mosque's supporters, however, have harshly criticized the country's new administration, saying it should have allowed the reopening of a controversial madrassa, or religious seminary, that became the focal point of last summer's confrontation.

Authorities described the Sunday blast as a suicide bombing, but it was not immediately clear whether the bomber arrived on foot or by some other means. There was no immediate claim of responsibility for the bombing, and local officials quoted a mosque spokesman, Mohammed Amir Siddiq, as condemning the attack.

The blast left a trail of wounded and dead police officers in bloodied blue uniforms, their helmets, caps and shoes scattered in the street along with metal shards, broken glass and debris. Some officers lost limbs in the blast. Ambulances rushed to the scene, as did hundreds of worshippers from the mosque and bystanders from the market.

Authorities had deployed hundreds of police to secure protests held to commemorate the raid's anniversary. But the large concentration of police left them vulnerable to attack.

Militants had made threats after the government late last month sent paramilitary troops to flush out militants from a tribal area close to Peshawar, the main city in northwest Pakistan. The deployment marked the first time the government, which took office in March, had moved militarily against the insurgents. Until then, the government had tried to negotiate with the militants.