The Iraqi Government sent thousands of troops into the badlands of Baghdad today at the start of a crackdown against insurgents who hold the capital in their thrall.

The operation, involving both Iraqi and United States forces, came the day after a surprise visit to Baghdad by President Bush, who told Nouri al-Maliki, the new Iraqi Prime Minister, that the country's future was "in your hands".

Tens of thousands of Iraqi police and soldiers deployed through the city of six million people, searching cars and securing key thoroughfares. But they met resistance from gunmen who who blocked roads with rocks and tree trunks and exchanged fire with soldiers in two parts of the capital.

Mr al-Maliki, a Shia Muslim, has been trying to reach out to Sunni Arabs to bring them into mainstream politics and also hopes to corner the foreign-led terror group al-Qaeda in Iraq after the death last week of its feared leader, Abu Musab al-Zarqawi.

At least four civilians were killed, however, when a bomb exploded in a parked car in northern Baghdad, apparently targeting a passing police patrol.

Clashes also broke out in the northern Sunni district of Azamiyah shortly before noon, with heavy exchanges of gunfire sending residents scurrying for cover. The clashes took place near the Grand Imam Abu Hanifa mosque, the holiest Sunni Muslim shrine in Iraq.

Most stores were closed in Azamiyah and the suburb of Dora, another insurgent stronghold where entire streets were deserted.

In central Baghdad and the predominantly Shia suburb of Karradah, hit by a series of deadly car bombs in the past week, Iraqi army soldiers patrolled on foot. Some were deployed at main intersections in pickup trucks with machine-guns mounted on their roofs.

Iraqi officials said that "Operation Forward Together" would involve 40,000 Iraqi and coalition forces in an effort to corner al-Qaeda and hit insurgents in Baghdad, even though similar operations have failed to stem the violence. On its first day, US forces kept a relatively low profile, although they patrolled parts of Baghdad in Humvees or Bradley fighting vehicles.

Mr Al-Maliki called on Iraqis to be patient with the security measures and promised that Iraqi forces would respect human rights and would not single out any ethnic or sectarian group. "We are only going to attack areas that are dens for terrorists," he told a press conference to launch the operation.

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