ISLAMABAD, Pakistan — Police officer Absar Ali was talking with colleagues near the busy Islamabad market when the suicide bomber attacked, rattling Pakistan’s usually quiet capital. The next thing the 37-year-old saw was “a bed of corpses.”

“Only two of us were standing. I fainted at the sight. My legs gave way under me, and I lost consciousness,” Mr. Ali said yesterday from his hospital bed, his feet bandaged.

The blast Sunday was one of Islamabad’s deadliest in recent memory, killing at least 18 people and wounding dozens. Mourners held funeral prayers for the slain yesterday, while the chief of the Interior Ministry, Rehman Malik, said investigators found a severed head, possibly the attacker’s.

Yesterday evening, six small explosions struck the southern city of Karachi, wounding at least 37 people and intensifying unease in Pakistan. The police chief of the Sindh province, Babar Khattak, said five men had been detained but gave no details on their identities or a motive for that attack.

Speculation linked the Islamabad blast to the first anniversary of the army’s crackdown on Islamic militants at the Red Mosque. A conference at the mosque drew thousands Sunday, and a mosque official condemned the bombing, one of many aimed at security forces in Pakistan.

Political leaders, too, decried the blast. The prime minister of Pakistan, Yousuf Raza Gilani, said it was “against humanity,” while President Musharraf, whose alliance with America in the war against extremists has fueled resentment among his countrymen, urged resilience.

The head of the main ruling party, Asif Ali Zardari, insisted the attack “would only strengthen the resolve of the government to confront the militants and extremists head on.” But the attack also promised to raise more concerns about government moves to negotiate with militants.

The U.S. Embassy advised American citizens not to go to the Red Mosque neighborhood for fear of more violence.

Pakistani officials were still trying to determine the identity of the bomber.