Thunderous explosions and dense black smoke swirled through the center of Baghdad on Monday when at least two car bombs ripped apart a crowded marketplace, setting off secondary explosions and killing at least 71 people, police said. Another bombing nearby killed at least nine. The blasts shattered the city center on the first anniversary, according to the Muslim lunar calendar, of the bombing last year of the important Shiite Golden Dome shrine in Samarra, north of the capital. That attack by al-Qaida in Iraq militants set off the torrent of sectarian bloodletting that has turned Baghdad and much of central Iraq into a battleground. A column of smoke hundreds of meters wide billowed 300 meters into the air above the market near the east bank of the Tigris River and near the Central Bank building. Ambulances and pickup trucks rushed many of the nearly 165 wounded to nearby al-Kindi hospital in the largely Shiite region that has been hit be a series of deadly bombings since the first of the year. The worst carnage occurred about 12:25 p.m. when two parked car bombs exploded shortly after the Iraqi government called for a 15-minute period of commemoration for the bombing of the golden domed shrine in Samarra a year ago. The bombs struck within a minute of each other, targeting two buildings about 200 meters apart. One of the cars was parked near the entrance to a parking garage under one of the buildings. Shops and stalls were obliterated and billowing smoke blackened the entire area on a beautiful sunny day in Baghdad. Debris and clothing mannequins were scattered in thick pools of blood on the floor of the warehouse-type building while men tossed plastic chairs onto piles. Two men carried the limp body of one of the victims, while small fires burned in the rubble on the street outside the building. A shop owner whose business was set on fire said one of the cars was parked in a garage under a two-story market called Al-Arabi, next to the Iraqi central bank. Mohammed Najaim said flames were coming out of the garage, which holds hundreds of cars. About half an hour earlier, a bomb hidden in a bag exploded in a crowded area near a popular take-away falafel restaurant in the Bab al-Sharqi area, not far from Shorja, police said, adding that 19 people also were wounded in that blast. The attacks, which occurred in busy market districts on the east side of the Tigris River, came despite stepped up security in the capital as US and Iraqi forces have launched a new operation aimed at stopping the sectarian violence that has been on the rise since the Feb. 22 bombing of the mosque in Samarra. The anniversary fell on Monday according to the Islamic lunar calendar. The lunar month is never longer than 30 days or shorter than 29. The beginning of each lunar month is set by religious authorities. One 38-year-old Shi'ite man said the blasts were clearly timed to coincide with the commemoration of the bombing in Samarra. Other people in the area screamed, "Where is the government?" "Where is the security plan?" "We have had enough?" "We have lost our money and goods and our source of living." Some storekeepers were trying to salvage merchandise while others were taking their money in small bags. Police and soldiers were deployed in force, and armed men in civilian clothes were searching and questioning people coming to the scene. Elsewhere in Baghdad, some roads and bridges in Baghdad were cordoned off after Prime Minister Nouri al-Maliki called for a 15-minute sit-in to commemorate the bombing of the al-Askariya or Golden Dome shrine in Samarra, 95 kilometers north of Baghdad. Al-Maliki issued a statement calling on government offices and all citizens to "chant `God is great' in all the mosques, and to ring bells in all the churches" for the Samarra anniversary. "The explosion of the holy shrine pushed the country into blind violence, in which tens of thousands of innocents were killed. No one knows but Allah when this tragedy will be over," Grand Ayatollah Ali al-Sistani, Iraq's top Shi'ite cleric, said in a statement issued Monday before the bombings. Al-Sistani urged the Iraqi government to rebuild the shrine, whose golden dome was partially torn off by last year's blast. The compound has since been locked and guarded by Iraqi police. But he also called for restraint among those observing Monday's anniversary. "We call on the believers to express their emotions but to be cautious and act disciplined, and not to do anything to hurt our brothers the Sunnis, as they are not responsible for this awful crime," he said. About 16,000 demonstrators flooded the main street of the southern city of Karbala, 80 kilometers south of Baghdad, marching toward two Shi'ite shrines there. Participants rallied with placards reading, "No to terrorism" and "Iraqis are one people, whether Shi'ite or Sunni." Hundreds of policemen guarded the area, and no violence was reported. Muqtada al-Sadr, the anti-American cleric who commands one of Iraq's most notorious Shi'ite militias, the Mahdi Army, was scheduled to speak to supporters in the holy city of Najaf later Monday. In 2006 alone, the United Nations reported that 34,452 civilians were killed in violence that has left Iraq battered and divided along sectarian lines. On Monday, Iraqi President Jalal Talabani called the shrine bombing that sparked a year of killings "a crime against humanity and Islam together." "This horrible crime drives us to toward more solidarity and brotherhood," Talabani said in a speech in Baghdad. "We will stay with you until we accomplish a secured, democratic, federal and stable Iraq away from the darkness of terrorism, dictatorship." Iraqi Vice President Adil Abdul-Mahdi accused al-Qaida of using the Samarra bombing to "stir sectarianism" and urged Iraqis to rebuild their country. "We should not stand thwarted. All Iraqis - Arabs, Kurds, Turkomen and others - have to move forward to rebuild the new Iraq after it was ruined for decades," he said. "There is nothing in front of us except to share society together."