A series of bombings hit Baghdad on Thursday, killing at least 63 people in the first big assault attack on Iraq's capital since a sectarian crisis erupted within its government just days after the U.S. troop withdrawal.

The apparently coordinated bombings were the first sign of a violent backlash against Shi'ite Muslim Prime Minister Nuri al-Maliki's move to sideline two Sunni Muslim rivals, raising the risk of a relapse into the sort of sectarian bloodletting that drove Iraq to the brink of civil war a few years ago.

At least 18 people were killed when a suicide bomber driving an ambulance detonated the vehicle near a government office in Baghdad's Karrada district, sending up a dust cloud and scattering car parts into a kindergarten, according to police and health officials.

We heard the sound of a car driving, then car brakes, then a huge explosion, all our windows and doors are blown out, black smoke filled our apartment, said Maysoun Kamal, who lives in a Karrada compound.

In total at least 63 people were killed and 194 were wounded in more than ten explosions across Baghdad, security and police sources said. Most of the targeted districts were Shi'ite.

Iraqi officials quickly branded the attacks a political message sent during the current crisis.

The timing of these crimes and the places where they were carried out confirm to all... the political nature of the targets, Maliki said in a statement.

Two roadside bombs struck the southwestern Amil district, killing at least seven people and wounding 21 others, while a car bomb blew up in a Shi'ite neighbourhood in Doura in the south, killing three people and wounding six, police said.

More bombs ripped into the central Alawi area, Shaab and Shula in the north, all mainly Shi'ite areas, and a roadside bomb killed one and wounded five near the Sunni neighbourhood of Adhamiya, police said.

An old woman wrapped in black was yelling and calling for her husband lost under the rubble after two bombs struck a wholesale vegetable market where they both worked.

I cannot find my husband, I don't know if they took him out or not, I don't know, she said.

Violence in Iraq has ebbed since the height of sectarian slaughter in 2006-2007, when suicide bombers and hit squads targeted Sunni and Shi'ite communities in continual attacks that killed thousands of people.

Iraq is still fighting a stubborn, lower-grade insurgency with Sunni Islamists tied to al Qaeda and Shi'ite militias, who U.S. officials say are backed by Iran, staging daily attacks.

We live in complicated circumstances, a complicated political scene and there is a conspiracy on Iraq from within, Baghdad security operations spokesman Qassim al-Moussawi said.

U.S. TROOPS OUT ONLY DAYS AGO

The last few thousand American troops left Iraq over the weekend, nearly nine years after the invasion that toppled Sunni dictator Saddam Hussein. Many Iraqis had said they feared a return to sectarian violence without a U.S. military buffer.

Just days after the withdrawal, Iraq's fragile power-sharing government is grappling with its worst turmoil since its formation a year ago. Shi'ite, Sunni and Kurdish blocs share out government posts in a unwieldy system that has been impaired by political infighting since it began.

This week, Maliki called for the arrest of Sunni Vice President Tareq al-Hashemi on charges he organised assassinations and bombings, and he asked parliament to fire his Sunni deputy Saleh al-Mutlaq after he likened Maliki to Saddam.

Hashemi, who has denied the accusations, has taken refuge in Iraq's semi-autonomous Kurdish region where he is unlikely to be handed over to the Shi'ite-led government in Baghdad.

The moves against the senior Sunni leaders have fanned sectarian fires anew because Sunnis fear the prime minister wants to consolidate Shi'ite domination over the country.

Iraq's Sunni minority have felt marginalised since the rise of the Shi'ite majority in Iraq after the 2003 invasion. Many Sunnis feel they have been shunted aside in the power-sharing agreement that Washington touts as a young democracy.

Thursday's attacks were the first major assault in Baghdad since November, when three bombs exploded in a commercial district and another blast hit the city's western outskirts on Saturday, killing at least 13 people.

In October, bomb attacks on a busy commercial street in northeastern Baghdad killed at least 30, with scores wounded.

(Additional reporting by Aseel Kami; Writing by Patrick Markey and Rania El Gamal; Editing by Mark Heinrich)