* MP's brother and bodyguards among those killed
* Sunnis have staged protests in Anbar for a year
* Iraq at its most violent since 2006-7 sectarian bloodshed
Iraqi security forces arrested a prominent Sunni Muslim lawmaker and supporter of anti-government protests in a raid on his home in the western province of Anbar, sparking clashes in which at least five people were killed, police sources said.
The violent arrest of Ahmed al-Alwani is likely to inflame tensions in Sunni-dominated Anbar, where protesters have been demonstrating against what they see as marginalisation of their sect by Prime Minister Nuri al-Maliki's Shi'ite-led government.
Alwani belongs to the Sunni-backed Iraqiya bloc and has been a strong critic of Maliki and an influential figure in the protest movement.
Police sources said a two-hour firefight broke out on Saturday when bodyguards and members of Alwani's tribe resisted police and soldiers who went to arrest Alwani on charges of "terrorism" from his house in the centre of the city of Ramadi.
They said those killed in the fighting included three of Alwani's bodyguards, his sister and his brother.
"Army troops with police special forces were trying to arrest Alwani from his house, but fierce fighting erupted. Five bodies, including one woman, were taken to Falluja hospital," one police source said.
No members of Alwani's family could immediately be reached to give their version of events. Parliament speaker Usama al-Nujaifi, a Sunni, called the operation a "blatant violation" of Iraq's constitution and a "dangerous precedent".
Lieutenant-General Ali Ghaidan, commander of Iraqi ground forces, told state television that security forces had also tried to arrest Alwani's brother Ali, whom he accused of involvement in attacks that killed Iraqi soldiers in Anbar.
Ali was killed in the fighting, as well as one Iraqi soldier, Ghaidan said.
"We treated Ahmed al-Alwani well. We told him that we had a warrant for his arrest, and arrested him," he said, adding that two of Alwani's bodyguards were wounded in the operation, which he said was carried out in accordance with the law.
Violence in Iraq is at its worst levels since 2006-7, when tens of thousands of people were killed in fighting between Sunnis and Shi'ites. Bombings, shootings and suicide attacks, many staged by al Qaeda militants, are a near-daily occurrence.
The Iraqi army this week launched a major operation in desert areas of Anbar, a province that makes up a third of Iraq's territory and is populated mainly by Sunnis, to flush out al Qaeda-linked militants who have gained a foothold there.
Another potential flashpoint is around Sunni protests that have been held in Anbar since December last year. In a statement on state television on Friday, Maliki said it would be the "last Friday" the protests and sit-ins would be allowed to continue.
The military campaign and the arrest of Alwani may help Maliki boost his popularity in the eyes of Shi'ites who had called on him to crack down on al Qaeda and on politicians they accused of encouraging violence. Elections are due in April.
Many Sunnis in the region are meanwhile likely to see Alwani's arrest as another example of what they portray as a crackdown against minority Sunni leaders.
A photo of the arrested lawmaker posted on Maliki's Facebook page purportedly showed Alwani after the arrest with a white bandage wrapped around his abdomen.
Authorities imposed a curfew in Anbar province after the arrest, security sources said. A convoy of army tanks, armoured vehicles and Humvees began to arrive in Ramadi by midday.
But scores of people took to the streets carrying guns to defy the curfew, a Reuters witness said.
Adnan al-Muhanna, leader of the al-Bu Alwan tribe, a major Sunni tribe in Anbar to which Alwani belongs, called on the government to release Alwani within 12 hours.
"Otherwise we will not be able to restrain the angry masses and that could lead to undesired consequences," he told Reuters.
Anbar's provincial council held an urgent session and called for Alwani's release and an investigation into the death of his brother, a copy of a statement seen by Reuters said.
The statement also called on all parties to exercise restraint and urged parliament to intervene and defuse tension.