Pakistani artillery and helicopter gunships pounded militant positions Sunday close to the Afghan border, an extremist stronghold where a rebel leader blamed for the assassination of former Prime Minister Benazir Bhutto is believed to be hiding, officials and witnesses said.

The strikes came after authorities said they had arrested a 15-year-old boy alleged to have been involved in the Dec. 27 slaying of Bhutto, an opposition leader critical of rising Islamic extremism in the country.

Meanwhile, Sunni extremists fired small arms and mortars at a Shiite procession commemorating Ashoura, a Shiite Muslim holiday that is often scarred by sectarian violence, the military said in a statement. Nine civilians and three security troops were injured in the incident in northwestern Hangu town, which ended after troops fired tear gas from a helicopter, it said.

Two civilians were killed and five others wounded in the attacks near the border close to the towns of Lhada and Makin in South Waziristan, said Fazal Subhan, a Makin resident. However, military spokesman Maj. Gen. Athar Abbas said there were no reports of casualties in the operations.

Fighting in South Waziristan in recent days has killed more than 100 soldiers and militants.

The violence, including suicide attacks that have killed hundreds in recent months, comes as the nuclear-armed country prepares for Feb. 18 elections that many predict will weaken President Pervez Musharraf's grip on power.

The central government has never had much control over South Waziristan, a tribal area where several top militants, including Baitullah Mehsud, a Pakistani accused by the government and the CIA of masterminding the killing of Bhutto, are believed to live.

On Saturday, officials said they had arrested the 15-year-old boy in northwestern Pakistan and that he had confessed to taking part in a plot to kill Bhutto in a gun and suicide bomb attack in the garrison town of Rawalpindi. It was the first arrest in the investigation.

A senior intelligence official said the boy told investigators that his five-person squad was dispatched to Rawalpindi by Mehsud. The official asked not to be named because he was not authorized to speak to the media. Mehsud, throughout a spokesman, denied any role in the killing.

Other than the incident in Hangu, Ashoura appeared to pass peacefully in the country of 160 million Muslims.

Tens of thousands marched and beat their bare backs with chains and blades, bloodying themselves in a sign of penitence. They said they were not worried about the possibility of attack.

"The procession is not something that could be curbed through fear of death," said Qaiser Abbas Zaidi, a retired civil servant in Rawalpindi. "People are slashing their heads with knifes and beating their chests in mourning. It means they are ready for death."

On Thursday, 11 people died in a suicide attack on a Shiite mosque in the northern city of Peshawar.

In the far southern city of Karachi, police detained five men on Saturday with explosives, detonators and a small quantity of cyanide they said was intended for attacks on Ashoura.

"With these arrests we have foiled major attacks," police chief Azhar Farouqi said, adding that the militants may have wanted to put the cyanide into the municipal water supply.

The rising violence has undermined the popularity of Musharraf, a key ally in the U.S. war on terrorism.

He left Sunday for Europe on an eight-day trip to meet leaders and attend the World Economic Forum in Switzerland. He said the trip was aimed at "improving Pakistan's image and removing certain misperceptions," but did not elaborate, according to the state-run news agency.