BAGHDAD, Iraq - British troops killed seven gunmen in a raid on a renegade police unit in southern Iraq on Monday, and a suicide bomber attacked a police checkpoint west of Baghdad, killing three policemen and injuring three other officers.

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The bombing happened at the entrance to a university in the city of Ramadi, the capital of Anbar province, a stronghold of the Sunni-dominated insurgency and one of the most dangerous places in Iraq for US troops. A curfew was imposed in the area.

Christians attended Christmas Mass in some churches in Baghdad and in northern Iraq, which is home to most of Iraq's 800,000 Christians. Some in Baghdad stayed at home, fearing they could fall victim to Iraq's violence. Christians are on the fringes of the conflict, which mostly involves Shiites and Sunnis, but they have been targeted by Islamic militants.

"I hope next year will bring good things and unite all Iraqis because there is no difference between Christians and Muslims," said Abu Fadi, a worshipper who does not use his Christian name because he fears for his safety. "May God bring relief from this."

Backed by tanks and armored personnel carriers, British troops conducted a raid in the city of Basra after receiving intelligence that a renegade group of Iraqi police might execute its prisoners at its station, the British military said. British troops have periodically conducted raids in an effort to root out corruption in the local police force, which has been infiltrated by Shiite militias.

Leaders of the police station's serious crimes unit were suspected of involvement with local death squads, and seven were apprehended three days ago in raids, said Lt. Jenny Saleh of the British Royal Navy in Basra.

"We had intelligence to indicate that the serious crimes unit would execute its prisoners in the coming days, so we decided to intervene," Saleh said without elaborating.

British troops were fired on as they approached the station and killed seven gunmen, said Maj. Charlie Burbridge, a British military spokesman.

British and Iraqi forces transferred all 76 prisoners at the police station to another detention facility in downtown Basra, he said. Once the station was evacuated, British forces destroyed it with explosives.

"We identified the serious crimes unit as, frankly, too far gone. We just had to get rid of it," said Burbridge, who alleged that its members were involved in tribal and political feuds in the mostly Shiite south, rather than sectarian violence of the kind sweeping the mixed neighborhoods of Baghdad.

Mohammed al-Askari, a spokesman for Iraq's Defense Ministry, said the operation was coordinated with the Iraqi government.

"Multinational forces got approval for this raid from this ministry and with participation of the Iraqi army," he said.

Britain has 7,000 troops in Iraq, mostly based around Basra. British officials have said they expect to withdraw several thousand troops from Iraq next year. More than 120 British personnel have died in Iraq since the US-led invasion in March 2003.

In Baghdad, a spokesman for Iraqi President Jalal Talabani said Monday that US troops detained two Iranians who were in Iraq at his invitation. The detentions follow US accusations that Iran is supplying aid and training to Shiite militias in Iraq, as well as technology for roadside bombs. Iran says it only has political and religious links with Iraqi Shiites, but is believed to be expanding its role in Iraq partly to counter US influence in the region.

"The president is unhappy about it," Hiwa Osman, Talabani's media adviser, said of the detentions of the Iranians.

Osman had no further details. The US military said it had no comment.

Late last month, Talabani visited Iran for two days of talks with government officials to seek their support in quelling the raging sectarian violence in Iraq. Iran, a Shiite Muslim country, has considerable influence among Iraq's Shiite majority ¡ª elements of which have been blamed for the bulk of the recent attacks.

In other violence Monday, a suicide bomber blew up at an Iraqi army checkpoint south of Ramadi, and clashes then erupted between gunmen and soldiers, a police officer said on condition of anonymity. Mortars exploded in the area, he said.

A sniper shot and killed a police commando in Samarra, northwest of Baghdad, police said. The Feb. 22 bombing of a Shiite shrine in Samarra set off reprisals against Sunni mosques and clerics in a tit-for-tat, sectarian war that dramatically increased the violence in Iraq this year.