A suicide car bomber and multiple roadside bombs hit Baghdad's mainly Shi'ite areas on December 22 in the first attacks on the capital since U.S. troops withdrew from Iraq on December 18.

In a sign of growing tensions within the government itself, Shi'ite Prime Minister Nuri al-Maliki has ordered the arrest of Sunni Vice President Tareq al-Hashemi and asked parliament to fire Sunni Deputy Prime Minister Saleh al-Mutlaq.

The U.S.-based SITE Intelligence Group said the Islamic State of Iraq, an umbrella group for al Qaeda-linked insurgents, had claimed responsibility for the attacks in a statement posted on Islamist websites Monday.

ISI said it had carried out the attacks in support of Sunni prisoners. "The operations were distributed between targeting security headquarters, military patrols...and eliminating the heads of unbelief from amongst the security, military and administration leaders of the Green Zone (Iraqi) government," it was quoted by SITE as saying.

In Thursday's single biggest attack, at least 18 people were killed when an attacker driving an ambulance detonated the vehicle near a government criminal investigation office in Baghdad's central Karrada district.

Hashemi has been formally charged with running death squads targeting Iraqi government and security officials. He has denied all charges which he says were "fabricated."

Overall violence in Iraq has dropped since the peak of sectarian fighting in 2006-07 but bombings and killings still occur almost daily.

Al Qaeda in Iraq has been weakened by deaths of leaders but there are fears the group will try to regroup and strengthen its presence following the withdrawal of U.S. troops almost nine years after the invasion that toppled Saddam Hussein.