Pakistani police released a photograph on Saturday of a suicide bomber who killed at least 139 people, as opposition leader Benazir Bhutto worked out her next step after the bloody start to her comeback campaign.

The militant threat demonstrated to such devastating effect in Karachi on Friday raised fears over the prospects for a national election due in early January that is supposed to mark a transition from military-led to civilian-led democracy.

Prime Minister Shaukat Aziz said the general election would not be affected but government officials had indicated that campaigning could be restricted because of security worries.

Newspapers carried photographs of the head of the suicide bomber propped on a white sheet. The dead eyes stared blankly out of a chubby, unshaven face.

"The age of the suspect is between 20 to 25 and he looks to be a Karachiite," said a security official, who declined to be identified.

Police said at least 139 people were killed in Friday's attack and 325 were wounded. On Saturday, a car bomb killed four people in the southwestern province of Baluchistan.

Suicide bombings have multiplied since the army stormed the Red Mosque in the capital Islamabad to crush an armed student movement in July.

The United States and its allies want to see elections go-ahead in nuclear-armed Pakistan in the hope that a moderate, pro-Western government will emerge to fight the al Qaeda and Taliban threat and help Western forces stabilize Afghanistan.

Washington is believed to have quietly promoted an alliance between Bhutto and its ally President Pervez Musharraf, the army chief who came to power in a coup in 1999.

For now, Bhutto has put on ice plans to go to Larkana, a town 240 km (150 miles) northeast of Karachi, to pray at the tomb of her father, Zulfikar Ali Bhutto, Pakistan's first popularly elected prime minister, who was deposed and executed after another military coup three decades ago.

"We are observing a three-day mourning over the tragedy. The mourning will end on Sunday and then she will decide about her next plans," said party spokeswoman Sherry Rehman.

Angry Bhutto supporters burnt tires, threw rocks at cars and forced shops to shut in some Karachi neighborhoods on Saturday but there were no reports of casualties, police said.

The hundreds of thousands of supporters who turned out to greet Bhutto 10 years after she last held power, and eight years since she went into self-imposed exile, showed she retained more mass appeal than any other Pakistani politician.

LOOKING FOR CLUES

Investigators' main focus will be on who sent Friday's suicide bomber. Government officials have said the culprits were Islamist militants but they are uncertain which group.

Pakistani Taliban working with al Qaeda this month threatened to kill Bhutto, who has talked of working with Musharraf to fight militancy.

But Bhutto, at a news conference on Friday, said she had more to fear from unidentified members of the power structure who she described as allies of the "forces of militancy". She said she had given names to Musharraf and wasn't blaming the government.

There is speculation that Musharraf might share power with Bhutto after the elections. But there is opposition to any partnership from within the ruling Pakistan Muslim League, a collection of politicians who Musharraf pulled under his banner.

Meanwhile, Musharraf is struggling to keep Nawaz Sharif, the prime minister he ousted and exiled, out of the country. Sharif was deported to Saudi Arabia in September when he tried to end his exile.

Saudi Arabia is putting pressure on Musharraf to let Sharif return, but Musharraf fears PML members will defect if their old leader is allowed back, according to diplomats.

Musharraf, who has pledged to quit the army if his October 6 re-election by parliament is ratified, is also trying to keep the Supreme Court at bay.

The court, regarded as hostile since Musharraf tried to sack its top judge in March, is hearing a number of cases against the government, including challenges to Musharraf's eligibility to have stood for re-election while army chief.