Nouri al-Maliki, the embattled Iraqi Prime Minister, said today that he would release 2,500 prisoners being held with no clear evidence against them or who had been mistakenly detained.

Mr al-Maliki announced the move in a televised press conference, saying that it would help "national reconciliation".

"We hope they will abide by not violently objecting to the political process. This is a strong move which will encourage others," he said, in a clear reference to the minority Sunni community, which forms the backbone of the Iraqi insurgency.

Mr al-Maliki said that the prisoners would be released from US-run detention centres and Iraqi custody. He added that a committee was being formed to review the status of all prisoners in Iraq but at the same time he would use force against those who continued violent acts. The first 500, from a total of 28,799 detainees being held in Iraqi and US prisons, will be freed on Wednesday.

"Those who will be released will be people who are not Saddam Hussein loyalists or terrorists or anyone who has Iraqi blood on their hands," he said. "Those who committed killings or bombings will not be released and they will be banned from being released."

The prisoner release comes against the background of continued sectarian violence, much of it clearly designed to provoke retaliation and push the country into all-out civil war.

Police today found nine severed heads in fruit boxes near the volatile city of Baquba, northeast of Baghdad, the second such discovery in less than a week. The boxes containing the heads - all from men - were discovered by a highway in the village of Hadid.

The heads were transferred to the city morgue and an investigation was under way, according to the Joint Cooperation Center, which is run by Iraqi and US forces. Iraqi police also found eight severed heads in the village on Saturday.

At least nine people were killed in other incidents across Iraq today, including two killed during a mortar attack on the interior ministry building.

Yesterday, gunmen dressed in Iraqi police uniforms launched an audacious mass kidnapping in broad daylight when they seized 50 people from a busy street in Baghdad.

The abduction appeared to be part of the growing sectarian violence between rival Sunni and Shia Muslim groups which has left hundreds dead in mixed neighbourhoods in and around Baghdad this year.

The attack occurred near the city’s bus stations, when a convoy of a dozen pick-up trucks, bearing the camouflage markings of the Police Commando unit, sealed off a corner of the Salihiya area popular with travel agents for those going by road to Jordan, Lebanon and Syria.

"It took them about five minutes to take people away. One, two, three, four — one after another," said Hamza Ali, an eyewitness. "Those abducted appeared to have been taken at random from the street and included vendors, workmen and even some Syrian nationals.

"They took people randomly," said a shop owner. "They grabbed a father and his two children but left the mother on the street shouting."

There were fears that the raid may have been in retaliation for a grisly killing on Sunday, north of the city, when gunmen pulled over two minivans carrying teenage students. They forced the passengers off, separated Sunnis from Shias and executed 21 Shias who were first made to lie face down on the ground.

In the past Shia militias supported by Iran have been accused of kidnapping, torturing and executing Sunnis. There is also evidence that serving members of the security forces have been implicated.

Major General Adnan Thabet, the chief of the Iraqi national police, denied that any of his officers was involved. But the raid will erode confidence in the new government of Nouri al-Maliki.

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