A demonstration by unemployed men in central Baghdad turned into a riot.

Police fired in the air, as protesters threw rocks and set cars ablaze.

Up to 100 men gathered outside a police station, where they said they had been promised jobs after making payments to police officers.

One policeman told Reuters news agency that they opened fire only after demonstrators fired first, and there were unconfirmed reports that several people were wounded.

A BBC correspondent who witnessed the incident says it shows that the situation in Baghdad remains extremely volatile, despite coalition claims that security is improving by the day.

A much larger demonstration took place in Mosul, beginning outside an employment office.

Some stones were thrown, but the march was otherwise peaceful.

In ongoing attacks on coalition troops, a US soldier was killed in a roadside bomb in Saddam Hussein's home town of Tikrit on Wednesday.

Two other soldiers were injured in the blast, near the entrance to a US base in the town.

The death brings to 88 the number of American soldiers killed by hostile fire since President George W Bush declared an end to major combat operations on 1 May.

Corrupt

The police blamed the fracas in Baghdad on provocateurs, who, they said, incited the men to attack the building.

"We didn't shoot at the beginning. We think this is a democracy and they can express their point of view. But then they started firing," policeman Falah Hassan told Reuters news agency.

The violence lasted for about half-an-hour. US troops arrived at the scene later, and firefighters were called in to extinguish burning cars.

"They promised us jobs in July," said one man.

"All these policemen are corrupt," said another. "We gave them money to register our names as candidates [for jobs] and when we returned they said we have no business being here."

Bribes of $100 were mentioned as being demanded in return for a job, with the unemployment rate estimated at 50%.

But the protesters said that, despite being paid, the police authorities had given the posts to their relatives.

The incident happened outside the headquarters of the Facilities Protection Service (FPS), just north of the Palestine Hotel, the main base for foreign journalists in Baghdad.

The FPS provides security guards for Muslim shrines and other sites.