Mr Alwani is a prominent supporter of anti-government protesters
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Struggle for Iraq
Streets of death
Iraqi forces have arrested an MP, killing his brother and five of his guards during a raid on his home in the western city of Ramadi.
Ahmed al-Awlani, a member of the Sunni community, had backed protests against the mainly Shia government of Prime Minister Nouri al-Maliki and was reportedly wanted on terrorism charges.
Police said Mr Awlani's guards opened fire as officers arrived to detain him.
Another 18 people were wounded in the ensuing skirmish, an official said.
"Security forces attacked the residence of MP Ahmed al-Alwani in central Ramadi to arrest him this morning, sparking a battle with his guards with automatic weapons and rocket-propelled grenades," a police major told AFP news agency.
"Five of Alwani's guards and his brother were killed and eight others wounded, while 10 security forces members were also wounded," the major said.
The security forces had also been hoping to arrest Mr Awlani's brother, Ali, authorities said.
Unconfirmed reports suggest that a sister of the MP - a member of the Sunni-backed Iraqiya bloc - was also among those killed in the raid.
The reasons for Mr Alwani's arrest were not immediately clear. Associated Press news agency said he was wanted on terrorism charges.
He has also been a prominent supporter of a group of activists camped on a highway near Ramadi protesting against the perceived marginalisation and persecution of Iraqi Sunnis by the Shia-dominated government.
Prime Minister Maliki has reportedly threatened to shut down the camp, saying it has become a headquarters of al-Qaeda.
AFP quoted parliament speaker Osama al-Nujaifi, a Sunni, as strongly criticising the arrest, describing it as "treading on the core of the Iraqi constitution and a clear violation of its articles".
In a statement, Mr Nujaifi insisted MPs had legal immunity and that he was sending a parliamentary delegation to Anbar province to investigate the case.
However, the constitution says MPs may be arrested without parliament waiving immunity if they are caught committing a serious crime.
Mr Alwani's arrest comes as Iraq remains in the grip of sectarian fighting which has made this the deadliest year since 2008.
More than 8,000 people have been killed since January, many of them in fighting between Sunni and Shia Muslims.
The UN has called on Iraq's political leaders to co-operate to end the bloodshed, which has escalated since an army raid on a Sunni Arab anti-government protest camp near the northern town of Hawija in April 2013 killed dozens.
The government has made some concessions in an effort to placate Sunnis, including freeing some prisoners and raising the salaries of Sahwa militiamen fighting al-Qaeda, but the violence has not abated.