Former military ruler Pervez Musharraf was Tuesday disqualified from contesting Pakistani elections next month, crushing his ambition to "save" the troubled country just weeks after his return from exile.

Pakistan goes to the polls on May 11 for an election that will mark the first time a civilian government has handed over power at the ballot box after completing a full term in office in a country used to extended periods of military rule.

The Pakistan election campaign has got off to a lacklustre start and been marred by violence and Taliban threats. On Tuesday, a bomb targeted a convoy of the main opposition Pakistan Muslim League-N (PML-N) party, killing four people.

Officials disqualified Musharraf just a day after he unveiled his All Pakistan Muslim League (APML) party manifesto at a press conference overshadowed by questions about legal cases dating back to his nine years in power.

The retired general is on bail over the 2007 killing of former prime minister Benazir Bhutto and the 2006 death of a rebel leader from the region of Baluchistan, and for sacking judges when he imposed emergency rule in 2007.

But he told reporters Monday: "The only thing in my heart was to save Pakistan and now I am here I have the same commitment, that I will save Pakistan."

The Taliban threatened to assassinate him on the eve of his return to Pakistan on March 24, where he was welcomed by only a few hundred ardent supporters.

The 69-year-old applied to run for parliament in four seats but was rejected immediately from all but the northern district of Chitral, on the Afghan border.

Lawyers appealed against his approval in Chitral and on Tuesday a court official said Musharraf's nomination had been thrown out on the grounds that he violated the constitution in 2007.

Musharraf's team have vowed to appeal against the decision in the Supreme Court, which is also hearing a separate petition from lawyers demanding that Musharraf face trial for treason dating back to his 1999-2008 rule.

The general himself pledged to continue his mission.

"The selection bias and unwarranted activism shown by the election tribunal in rejecting my nominations papers from all four constituencies is not going to deter my resolve to help save Pakistan," he said in a statement.

His lawyer Ahmed Raza Kasuri said the decision was an insult to "an internationally known person" and would show the world "what democracy we have".

Commentators said Musharraf was finished, given the hostility in the Supreme Court to the man who dismissed the current chief justice in 2007.

"Politically he never had any future," said political analyst Hasan Askari.

"I think he miscalculated his position and his advisers really fooled him advising him to come back... I think he should sit back in Dubai and the UK, and write another book."

Election observers from the European Union were sent to constituencies around Pakistan on Tuesday to begin long-term monitoring work ahead of the polls.

Tuesday's bombing in the southwestern province of Baluchistan was the third deadly attack on politicians or political parties in as many days.

Sanaullah Zehri, head of the PML-N in Baluchistan, survived, but his son, brother, nephew and their guard were all killed, officials said.

"An improvised explosive device went off as Zehri, leading a convoy of more than 20 vehicles, left his home to campaign in Khuzdar," provincial home secretary Akbar Durrani told AFP.

There was no claim of responsibility, but the Pakistani Taliban have said they were behind two deadly bomb attacks in the northwest on Sunday and the killing of a candidate for the secular Muttahida Qaumi Movement on Thursday.

Zehri survived a murder attempt two years ago that was claimed by the rebel Baluch Liberation Front.

A candidate for the Pakistan People's Party of President Asif Ali Zardari told AFP he escaped a grenade attack at his home in Peshawar unhurt on Tuesday.

On Monday gunmen killed two election campaigners for an independent candidate running in the lawless northwestern tribal regions.

Pakistan's caretaker interim prime minister Mir Hazar Khan Khoso ordered an immediate tightening of security for all candidates in the wake of that shooting.