Iraqi insurgents killed more than 30 people in double attacks in Baghdad today, including a bomb, gun and mortar raid on a police station in which they slaughtered 16 policemen and released dozens of prisoners.

The police massacre and bombing of a Shia mosque brought to a bloody end a month-long lull in major attacks in the capital.

The quiet period coincided with the huge US-led assault on Fallujah, raising fears that guerrillas who escaped the invasion have regrouped and readied themselves for an escalation of violence in the run up to elections on January 30.

A US military spokesman, Lieutenant Colonel Jim Hutton, said that insurgents in 11 cars first attacked the station with rocket-propelled grenades and small arms fire.

Gunmen then stormed the building itself, killing 16 policemen, looting weapons, releasing prisoners and torching several cars.

During the assault the attackers set free around 50 prisoners before killing the remaining police in cold blood, and torching police pickup trucks, sending clouds of smoke into the air. So confident of success were they, said witnesses, that they even brought their own camera crew.

"I saw about 30 masked men turn up in saloon cars carrying rifles and they had a guy with a video camera with them," said Ahmed Abu Hind, who lives next to the station.

"He was one of the first who jumped outside the cars and he was the last to leave.

"It took them about 30 minutes to take over the place while some of them stayed outside killing any policeman who tried to escape. Then they slaughtered the policemen who couldn't leave the place, and we could see them taking out all the guns and ammunitions from the place and loading it into their cars."

The attack follows a familiar pattern of targeting Iraq's police, army and National Guard units, many of whom are too terrified to resist.

The attack on the Shia Hameed al-Najar Mosque in Baghdad's Sunni-dominated Adhimiya district came shortly after daybreak, when a mortar blast was heard, closely followed by a car bomb that exploded killing 14 people and wounding 19, police said.

"The first blast happened just as worshippers were leaving the mosque after dawn prayers. Everyone in the area rushed to help them," said one local man sweeping up broken glass.

"Then a few minutes later, a car blew up the whole crowd."

The attack is thought to be designed to provoke civil war between Iraq's Shia Muslim majority and its 20 per cent Sunni population, who have long dominated the country but look set to lose power in next year's elections.

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