PESHAWAR, Pakistan — A suicide bomber struck an election rally in volatile northwest Pakistan Saturday, killing at least 20 people and wounding more than 45, officials said.

In southern Pakistan, the campaign for Feb. 18 parliamentary elections moved back onto the streets with Benazir Bhutto's widowed husband urging about 100,000 supporters to help him "save" the country. It was the first major campaign rally by the opposition leader's party since her December assassination.

In the capital Islamabad, riot police used water cannons and tear gas against hundreds of lawyers protesting the detention of the deposed chief justice.

The violence underscored the deep tensions in Pakistan as it heads toward elections meant to restore democracy after years of military rule. The campaign has been overshadowed by Bhutto's killing in a suicide attack six weeks ago.

In the northwestern town of Charsadda, a suicide bombing ripped through a rally of the Awami National Party, a secular, ethnic Pashtun group, killing at least 20 people and wounding 45, officials said. Mohammed Khan, a local police official, said two policemen were among the dead and four or five children were killed or injured.

Abdul Waheed, 22, who suffered burns from the blast, told The Associated Press from his hospital bed in the nearby city of Peshawar that the bomb went off as an Awami National Party member was reciting verses from Quran, Islam's holy book. He said he did not see the bomber.

"I only heard the blast and cries and then something hit me and I fell down," he said.

TV footage of the aftermath showed the hall, located in the sprawling residence of a party activist, littered with bloodstained clothes, police caps and wrecked chairs. Khan said police had found the severed head of the suspected bomber.

Interior Minister Hamid Nawaz said the attack showed that militants in the northwest, where Taliban and al-Qaida have sanctuaries near the Afghan border, were targeting both government and opposition parties.

"They are against everyone," he said.

At the rally for Bhutto's Pakistan People's Party, police estimated 100,000 people turned out in the historic southern town of Thatta — by far the biggest rally in the election campaign so far. Many thousands spilled onto the streets outside as the stadium was full and about 2,000 officers provided security.

Striving to fill the void left by the former prime minister, Bhutto's husband Asif Ali Zardari urged followers to push for victory in the vote and realize Bhutto's vision of providing jobs and ending poverty.

"I have the responsibility to save Pakistan," Zardari told the crowd.

Bhutto's party is widely expected to benefit from the unpopularity of U.S.-backed President Pervez Musharraf and sympathy over her assassination.

Zardari claimed Bhutto had been murdered by an establishment that she wanted to change.

"That is why they were against us," Zadari said. "If they try to stop me, I will destroy them and I hope you people will support me."

The government has rejected allegations that intelligence agents or radicals in the ruling party allied to Musharraf plotted to kill Bhutto. The government has said the attack was orchestrated by a top Taliban commander with links to al-Qaida.

Zardari is striving to unite the party and dispel public doubts over his track record. He was labeled "Mr. 10 Percent" over his alleged pilfering of state funds and demands for kickbacks during Bhutto's two administrations in the late 1980s and 1990s.

Pir Bakhsh, a 24-year old laborer, said that Zardari's reputation was not good but that love for Bhutto "compels us to attend this rally."

A vast portrait of Bhutto alongside a picture of her 19-year old son Bilawal dominated the backdrop of the stage where Zardari spoke. Bilawal, currently studying at Oxford University, was appointed party chairman after her death, but Zardari is the de facto leader.

The rally came a day after Scotland Yard released its findings that a suicide bomb, not a bullet, killed Bhutto after an election rally in the northern city of Rawalpindi on Dec. 27. That supported the conclusion of the Pakistan government, but is still disputed by Bhutto's party, which maintains she was shot.

Despite the party's support base among Pakistan's rural poor — particularly in her native Sindh province where Thatta is located — fear over rising militant violence and apathy over the nation's elitist politics has dampened public enthusiasm for the election.

In Islamabad, hundreds of riot police clashed with about 1,500 black-suited lawyers who marched on the residence of Iftikhar Mohammed Chaudhry, the deposed chief judge of the Supreme Court who was fired by Musharraf three months ago and remains under house arrest.

After the lawyers tried to breach barbed-wire barricade, police drove them back by firing tear gas and shooting water cannons from a fire truck. They also charged the crowd and beat protesters with batons.

Several protesters, including Athar Minallah, a prominent Supreme Court lawyer, were arrested. He told The Associated Press by cell phone from a police van that he had been hit on the head by police.

Earlier Saturday, Pakistan's Bar Council had announced a nationwide lawyers' boycott of courts until elections. The move is part of a campaign to pressure the government to restore the chief justice and 60 other top judges who were sacked by Musharraf on Nov. 3 ahead of a key Supreme Court ruling on the legality of his re-election as president.