The attempt on the life of Ayatollah Abdelaziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq (Sciri), came five weeks before the country’s elections — plunged into fresh chaos yesterday as the main Sunni party said that it would boycott them.

The ayatollah had assumed Sciri’s leadership last year when his brother was killed in a car bombing in Najaf, and yesterday party officials blamed the alliance of diehard Saddam loyalists backed by al-Qaeda thought to have carried out the earlier attack. Saad Jowad, a Sciri official, said: “I suspect that the people who were with Saddam, especially his security forces, and supported by Zarqawi, were behind this attack.”

The explosion took place at 9.30am as Ayatollah al-Hakim, who once headed Sciri’s armed wing, was in a regular morning meeting in Sciri’s main offices by the Tigris in a house that once belonged to Tariq Aziz, Saddam’s Deputy Prime Minister. Five Sciri guards were killed as well as several passers-by, Mr Jowad said, emphasising that no one inside the building was hurt, although all the windows were blown out.

Ayatollah al-Hakim heads the list of the Unified Iraqi Alliance, the main Shia bloc running in the January 30 elections, which has been endorsed by Iraq’s leading cleric, Grand Ayatollah Ali al-Sistani. While backed by Tehran, Sciri has also co-operated with the US-backed coalition in Iraq and has senior ministers in the interim Government.

The assassination of Ayatollah al-Hakim would have been a huge blow to the Shia list, after last week’s car bombings in the Shia cities of Karbala and Najaf. Iyad Allawi, the interim Prime Minister, has accused the al-Qaeda terrorist Abu Musab al-Zarqawi of focusing his attacks on Shia communities to foment sectarian strife and derail elections in which the Shia majority are expected to surge to power for the first time in Iraq’s history.

The ayatollah, who last week offered to provide 100,000 guards for polling stations, yesterday insisted that the Shia would not be provoked into violence, on the brink of electoral success. “We have chosen the path of non-violence and will stick to it,” he said. “The only ideology these people know is terror. We laid down our arms in favour of pluralism. If we wanted violence we would have responded a long time ago.”

But the run-up to the elections has been marked by bombings and assassinations more than by political campaigning, and there are doubts over whether a free and fair ballot is possible. Hoshyar Zebari, the interim Foreign Minister, visiting China, said that the vote might have to be postponed in some cities where security forces have proved unable to prevent rampant crime and incessant attacks.

“Elections in dangerous regions such as Mosul and around Baghdad may be delayed so that we can concentrate our security resources to destroy opponents’ capacity to target and intimidate people,” he said, contradicting the line in Baghdad, where officials have discounted such a delay.

Hardline Sunni officials, who feel disenfranchised by the rise of the Shia majority, have said that they would boycott the elections. This would undermine their validity and could further entrench the insurgency in the west and centre of Iraq that has cost the lives of hundreds of US troops.

Yesterday the leading moderate Sunni party, the Iraqi Islamic Party, which worked with the US postwar occupation, delivered its own blow to the elections. “We are withdrawing,” the party leader Mohsen Abdel Hamid said, accusing the interim Government of ignoring his “reasonable” appeal to defer the election by six months, but stopping short of calling for a general Sunni boycott.

The Iraqi electoral committee rejected a Washington-backed proposal to bypass the boycott threat by reserving some government positions for Sunni officials no matter who wins the poll. It called the suggestion “unacceptable” interference. Farid Ayar, a spokesman, said: “Who wins, wins. That is the way it is. That is the way it will be in the election.”

Yesterday an Iraqi National Guard officer claimed that 21 of his colleagues had been abducted in the west of the country. The officers were returning by bus across the desert to their base at Qaim, close to the Syrian border, it was claimed.

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