Several car bombs exploded in Shi'ite Muslim neighborhoods across Baghdad Sunday morning, killing dozens of people in blasts that tore into shops, restaurants and busy commercial streets.

Estimates of the dead ranged from 21 given by The New York Times to 37 according to the Associated Press.

No one has claimed responsibility, but Sunni Muslim insurgents have stepped up their attacks since the beginning of the year to undermine the Shi'ite-led government and trigger sectarian conflict.

The detonation of a parked car loaded with explosives in the sprawling Shi’ite district of Sadr City heralded the start of the attacks Sunday morning, the AP reported. Two more parked cars later exploded elsewhere in the neighborhood.

One blast tore off storefronts in Qaiyara district while another left the remains of a car and its twisted engine littered across a high street in the busy, commercial Karrada district packed with restaurants and shops, Reuters reported.

"I was buying an air conditioner and suddenly there was an explosion. I threw myself on the ground. Minutes later I saw many people around, some of them dead, others wounded," Habibiya district salesman Jumaa Kareem told Reuters, his jacket spattered with blood.

Sunday's blasts followed the assassination of a senior Iraqi army intelligence officer on Saturday, the latest in a wave of suicide bombings since January. No one claimed responsibility for that attack.

Many Iraq Sunnis feel they have been sidelined and unfairly targeted by security forces since the fall of Saddam Hussein and the rise of the Shi'ite majority through the ballot box.

As sectarian strife mounts, Sunni protesters have been staging weekly demonstrations and sit-ins since late December to rally against the government, which is led by Shi’ite Prime Minister Nouri al-Maliki. The protesters have rejected calls for violence and distance themselves from extremist groups such as Al Qaeda.

There are also concerns that Sunni insurgents could step up attacks ahead of provincial elections scheduled for April 20. The ballot would be the first nationwide vote since the U.S. troop withdrawal more than a year ago.

More than 10 suicide attackers have struck security forces, Shi'ite targets and a Sunni lawmaker since the start of January.

In the most recent attacks, a suicide bomber killed the head of the army's intelligence school on Saturday after storming his home in a northern town. Another suicide bomber killed 26 at a Shi'ite funeral at the start of the month.