BAGHDAD - Iraqi police arrested a man on Wednesday over the killing of a senior Sunni Muslim MP and human rights advocate who was shot dead in a Baghdad mosque last week.

Obaidi was killed one day after calling for an independent inquiry into torture and abuse of detainees in Iraq's prisons.

The arrest came after intelligence efforts led police to a house in the western Baghdad neighbourhood of Ghazaliyah, not far from Al-Yarmuk where Harith al-Obaidi was assassinated on Friday.

"We received information from one of our sources in Ghazaliyah that the group involved in the assassination were in one house," said Brigadier General Noaman Dakhil Jawad, the commander of the police's rapid intervention forces in Baghdad.

"We prepared our forces, we raided the house, and we arrested the criminal Ahmed Abed Oweiyed."

Jawad said Oweiyed was alleged to be the deputy commander of Al-Qaeda's military wing in Iraq.

"Through intelligence efforts, we tracked down the criminal who masterminded the killing," he said.

A teenage gunman shot dead Obaidi and his bodyguard in Al-Shawaf mosque in the predominantly Sunni neighbourhood of Al-Yarmuk after Obaidi led worship on the weekly Muslim day of prayer.

The gunman then killed three others and wounded 12 by throwing a grenade into a crowd, before killing himself.

Obaidi, born in 1966, was deputy chairman of parliament's human rights committee and head of the biggest Sunni bloc in parliament, the National Concord Front.

The day before he was killed, he called for an independent inquiry into torture and abuse of detainees in Iraq's prisons.

Prime Minister Nuri al-Maliki led mourners at Obaidi's funeral on Saturday, while Vice President Adel Abdel Mahdi, a Shiite Muslim like Maliki, described the murder as a "brutal crime."

Iraq has seen several political assassinations since the US-led invasion of March 2003.

In February, Islamic Party official Samir Safwat was killed outside his Baghdad home by gunmen in a car. A month earlier, two candidates standing in provincial elections held on January 31 were killed in Baghdad and Mosul.

The US-led invasion of Iraq in 2003 is viewed by critics as an 'act of aggression' that violated international law.

Subsequent US occupation policies caused the country to descend into almost total chaos, bordering on civil war.

An estimated 1.3 million Iraqis have been killed in Iraq as a direct result of the invasion, while millions more have fled the country.

Critics argue that the recent stability announced in the country should not excuse the 'crime' of invading Iraq, calling for the prosecution of the war's architects for 'crimes against humanity'.