BAGHDAD, Iraq — A series of explosions struck commercial areas in Baghdad on Saturday, killing at least seven people, a day after a suspected shoe bomber blew himself up inside one of Baghdad's most prominent Shiite mosques, killing 13 people.

The four blasts, all within two hours, dealt a new blow to a huge security operation by the Iraqi government to secure the capital.

Elsewhere, a suicide car bomber exploded his vehicle as it was being towed near a police checkpoint, killing four civilians, said Capt. Rashid al-Samarie. He said the bomber claimed his car had broken down and hired a tractor to tow it while he rode inside.

The attack happened in the mainly Sunni city of Mahmoudiya, 20 miles south of Baghdad, which was also hit by a mortar barrage that killed one civilian and wounded three others, all from the same family, al-Samarie said.

A soldier in the U.S.-led coalition also was killed Friday and two others were missing after an attack on a checkpoint near the town of Youssifiyah, 12 miles south of Baghdad, the U.S. military said.

The first attack in the capital occurred shortly before 10 a.m. when a mortar shell was fired at one of Baghdad's oldest markets in the predominantly Shiite suburb of Kazimiyah, Capt. Mohammed al-Waili said. He said at least four people were killed and 13 wounded.

About a half-hour later, a bomb left in a plastic bag struck an outdoor market for secondhand goods, killing two civilians and wounding 24, some seriously, police Lt. Ahmed Mohamed Ali said.

A roadside bomb also missed a police patrol about 10:40 a.m. in Karradah, a popular shopping area in downtown Baghdad, killing one civilian and wounding two, police Col. Abbas Mohammed said.

About 20 minutes later, a parked car bomb targeted an Iraqi army patrol near the Wathiq Square in the same neighborhood, killing one soldier and wounding 10 people, including two civilians, police Lt. Ali Mitab said.

The surge in violence has shattered a fragile calm imposed by the security crackdown launched Wednesday in the capital.

On Friday, a suspected shoe bomber targeting a Shiite imam who criticized Abu Musab al-Zarqawi blew himself up inside the Buratha mosque during the main weekly religious service, killing 13 people and wounding 28. That attack was carried out despite a four-hour driving ban intended to prevent suicide car bombs during Friday prayers.

The mosque's imam, a leading Shiite politician, blamed al-Qaida in Iraq. He said the terror group was trying to reassert itself after the death of its leader in a U.S. airstrike last week.

"Al-Qaida is trying to restore some respect after the killing of the terror leader Abu Musab al-Zarqawi by targeting one of the leading Shiite clerics, but they will fail," said the imam, Jalal Eddin al-Sagheer.

The imam, who was not injured, said the bombing came after guards found two pairs of shoes containing explosives outside the mosque. The guards entered the mosque and began searching everyone who had carried their shoes inside, he said.

When they approached the attacker, he detonated what would have been a third pair of explosives-laden shoes, he said.

But the Interior Ministry, noting the scale of destruction, suggested the attacker may have detonated a vest rather than shoes. Police Lt. Thaer Mahmoud said the attacker was indeed wearing a suicide vest.

The device contained metal balls and fragments, according to an Interior Ministry police officer who could not be named because he was not authorized to speak to reporters. Metal balls and fragments could fit into either shoes or a vest.

On Friday, Al-Jazeera aired an audio tape of a key insurgency leader calling al-Zarqawi's death a "great loss" but saying it will strengthen the militants' determination.