Former Pakistani prime minister Benazir Bhutto called on the government to seek foreign help in investigating last week's suicide bombing aimed at killing her on her return after eight years of exile.

Two blasts killed 139 people early on Friday as Bhutto travelled through the streets of Karachi packed with supporters welcoming her homecoming.

No one claimed responsibility for the attack but the government suspects al Qaeda-linked militants based on the Afghan border were responsible. They have carried out a string of similar attacks on the security forces and other targets, killing hundreds of people in recent months.

'We want the government of Pakistan to seek assistance from the international community. They have anti-terrorism expertise to investigate attacks of this nature,' she told reporters after attending a prayer ceremony for the victims.

While Bhutto also suspects militants were behind the attack, she has also alluded to the involvement of unspecified elements of security agencies in abetting such violence.

She has said she had written a letter to President Pervez Musharraf before her return to Pakistan citing at least three people she said could be involved in attacks against her. She did not give their names.

'Supporters of the militants and Al Qaeda are determined to stop democracy because they don't want a moderate majority to stand up,' she said.

Earlier in the day, Bhutto made an unannounced visit to a Karachi hospital to meet some of the more than 300 people wounded in the attack, a party spokeswoman said.

Hundreds of party members held prayers for the dead in Karachi.

Police said they had several leads and were making progress but they declined to give details. On Saturday, police released a photograph of the head of the suspected suicide bomber and offered a reward for anyone who could identify him.

The government has said the violence would not affect plans for a general election although campaigning might have to be curtailed because of the threat of violence.

Bhutto said a code of conduct would be prepared in consultation with other political parties in view of the deteriorating security situation.

'We will have to modify our campaign because of the suicide bombings but we are not going to stop our campaign to reach the public,' she said.

The US and its allies want to see elections go ahead in Pakistan in the hope that a moderate government will emerge to fight the Al Qaeda and Taliban threat and help Western forces stabilise Afghanistan.

In the North Waziristan region on the Afghan border, militants planted explosives and blew up the only girls' school in the main town, Miranshah, on Saturday night, residents said. Reuters