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Iraqi police, soldiers and even militiamen today took to the streets to enforce a daytime ban on private vehicles to prevent further violence in Baghdad and its outskirts.

The measure was intended to avert car bomb attacks during midday prayers, when large numbers of Muslims congregate for the most important religious service of the week.

Ibrahim al-Jaafari, the Prime Minister, also warned preachers not to incite hatred or violence in their sermons, threatening them with "severe measures" if they used "inflammatory" language.

The curfew was imposed by the Iraqi government hours after suspected al-Qaeda-linked gunmen last night raided Nahrawan, a small town near Baghdad, where they shot dead at least 19 Shia Muslims.

The raid was the latest and one of the most bloody incidents in a massive upsurge of violence since the Golden Mosque of Samarra, one of the holiest Shia shrines, was destroyed by bombs on February 22.

Violence has killed at least 500 people since the Samarra bombing, mainly Shias but also including 45 Sunni imams and mosque officials, by conservative estimates.

Among the dead in last night's massacre were Shia migrant labourers shot down at a brick factory, and nine killed at the local power station, which was wrecked.

The Nahrawan municipal council leader, Alaa Abdul Sahab al-Lamy, put the death toll higher, saying that 21 bodies were found at the brick factory and four at the power station. Many were found with single bullet wounds to the head and three of the victims were children, he said.

Today religious leaders were heeding the call for restraint. The fiery Shia cleric and militia leader Moqtada al-Sadr told followers to pray with Sunnis. In mosques across the country, preachers made similar calls for unity.

Mr al-Jaafari has used public shock at the Samarra bombing and its violent aftermath to rally Sunni and other leaders into resuming talks on forming a unity coalition government to help stem the violence. The main minority Sunni bloc has ended a boycott it called in protest at reprisal attacks on Sunni mosques.

US security officials fear that the strife may only worsen, and forces are on the alert for a follow-up "spectacular" attack by the group led by Abu Musab al-Zarqawi, a Jordanian Sunni militant linked to al-Qaeda."There’s been concern that Zarqawi might try to launch a large-scale attack," an official Reuters.

But hopes of forming a coalition government are being dashed by continued behind-the-scenes horsetrading. After Mr al-Jaafari hosted a late-night meeting last night of the main parties elected to parliament in December, political sources said that Sunnis, Kurds and other leaders were still pushing the dominant Shia Alliance to ditch him as premier.

"The negotiations will go on but we still insist on removing Jaafari," said an official in the Sunni Iraqi Accordance Front.

It was announced today that Iraq’s parliament is likely to hold its first session since being elected in December within the next 10 days, most probably on March 12.

"The meeting is around then. It could be on the 11th but so far what we are talking about is March 12," said an Iraqi government official. "If we do not meet by that time, we would be violating the law."

Contrary to the hopes of many political leaders, however, the first session of the full-term parliament is unlikely to see a deal between Iraq’s parties on a national unity government that could start to get a grip on the violence.

Government officials and other political sources said that arguments over posts, complicated by the sectarian violence and the efforts to unseat Mr al-Jaafari, meant only the job of parliament speaker was likely to be agreed by then.

Adnan Pachachi, a former foreign minister, would chair the first session under a rule handing that role to the oldest member of parliament, the sources said. He represents the secular Iraq List headed by Iyad Allawi, the former Prime Minister.

The assembly would then elect a speaker to serve for the coming four-year term. One possible candidate is a Sunni Arab leader, Tareq al-Hashemi of the Iraqi Accordance Front.

According to the Transitional Administrative Law, the new parliament has to meet within two weeks of the Electoral Commission announcing certified results. The parliament has already missed that deadline but the timetable allows for an extension by 15 days - a deadline which ends on March 12.

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