A suicide bomber in a vehicle blew himself up at a volleyball game in northwest Pakistan on Friday, killing at least 25 people, officials said, the latest in a long series of deadly attacks by al Qaeda-linked militants.

"We have confirmed a figure of 25 deaths. There are many more wounded who are being rushed to hospitals in private vehicles," a local police official, Habibullah Khan, told Reuters by telephone.

The bomber struck as young men in a village near the town of Lakki Marwat were playing volleyball in front of a crowd of spectators, including elderly residents and children, officials said.

Al Qaeda-linked Taliban militants are known to have strongholds in the region. The death toll could climb sharply as many people were buried under the rubble of nearby houses, officials said.

An attack on a sporting event is highly unusual, although militants have recently started bombing crowded civilians areas like markets to inflict mass casualties, spreading fear and chaos.

Khalid Israr, a senior regional official who spoke by telephone from a hospital treating blast victims said several people recalled seeing the bomber drive a vehicle onto the playing field and blow himself up. Dozens of anguished people, he said, were gathered at the hospital.

The bombing occcured on a day of protests against violence in Pakistan, an ally the United States needs to help stabilise neighbouring Afghanistan, where a Taliban insurgency is raging.

Despite major military offensives against their strongholds, al Qaeda-linked Taliban insurgents have managed to kills hundreds of people in bombings since October.

Pakistan's commercial capital Karachi nearly shut down on Friday as religious and political leaders called for a strike to denounce bloodshed after a suicide bomber killed 43 people at a religious procession this week.

The Taliban claimed responsibility for Monday's attack on a huge crowd of Shi'ite Muslims and threatened more bloodshed.

PRESIDENT UNDER FIRE

The prospect of increased violence comes at a trying time for President Asif Ali Zardari, who faces political heat because corruption charges against some of his aides may be revived.

The pro-American leader has vowed to defeat the Taliban and al Qaeda but they remain defiant, killing hundreds of people in bombings since October despite facing major government offensives.

In a sign of growing anxiety over security, the United Nations will withdraw some of its staff from Pakistan because of safety concerns, a U.N. spokeswoman said on Thursday.

On a visit to Karachi on Friday, Interior Minister Rehman Malik said militant groups were harming Pakistan.

"They are hired assassins. They are enemies of Pakistan. They are enemies of Islam," he told reporters.

While many Pakistanis object to the violence, they are also frustrated with the government's inability to stabilise the nuclear power.

Militants have become increasingly brazen in their bid to topple the government and impose their brand of Islam, including public floggings and executions for anyone who disobeys them.

The Karachi carnage illustrated their reach is growing, from bastions on the Afghan border to cities, including an attack on a mosque near the headquarters of the powerful military.

Karachi's streets were nearly empty on Friday. The stock exchange,, which normally operates on the first day of the year, was closed.

Police have arrested 18 people since riots triggered by the bombing destroyed hundreds of shops, costing Pakistan's biggest city an estimated 30 billion rupees ($356 million) in damages.

Police and paramilitary forces carried out patrols. But residents were taking no chances.

"We are already losing business and can't take the risk of going out today and opening our shops," said Saleem Ahmed, who sells electronics at one of the city's markets.

"If something happens or anyone comes and damages, say, one refrigerator or deep freezer, I will lose more money than what I would have earned the whole day, so I better stay home."