The United Nations said Tuesday that nearly 35,000 Iraqi civilians were killed in sectarian violence last year, nearly three times the number reported dead by the Iraqi government. Two back-to-back explosions, meanwhile, struck a used motorcycle marketplace in central Baghdad, killing at least 15 people and wounding 74, providing the latest example of the attacks faced by Iraqis on a daily basis. The first bomb was attached to a motorcycle in the market. As the curious gathered to look at the aftermath, a suicide car bomber drove into the crowd and blew up his vehicle. Authorities said at least three policemen were among the dead. The blast appeared to target the mainly Shiite neighborhood near the market but also was near the Sheik al-Gailani shrine, one of the holiest Sunni locations in the capital. Raad Abbas, a 26-year-old who received shrapnel wounds in the attacks, said he had gone to the market because the city had been quieter over the past two weeks. "Shortly after midday, I heard an explosion. Motorcycles were flying in the air, people were falling dead and wounded. "Some people were trying to help me when I heard the second explosion. The next thing I knew, I woke up here in the hospital," Abbas said from his hospital bed. The UN figure was released as Baghdad braces for a major security operation to be launched by the Iraqi government and US forces aimed at quelling the rampant sectarian violence that has been on the rise since the Feb. 22 bombing of a Shiite mosque in Samarra. Gianni Magazzeni, the chief of the UN Assistance Mission for Iraq in Baghdad, said 34,452 civilians were killed and 36,685 were wounded last year. Iraqi government figures in early January put last year's civilian death toll at 12,357. When asked about the difference, Magazzeni said the UN figures were compiled from information obtained through the Iraqi Health Ministry, hospitals across the country and the Medico-Legal Institute in Baghdad. "Without significant progress in the rule of law sectarian violence will continue indefinitely and eventually spiral out of control," he warned. Magazzeni said 6,376 civilians were killed violently in November and December - 4,731 of those in Baghdad, most as a result of gunshot wounds. He noted that was a slight decrease from the previous two-month period, during which UNAMI recorded a total of 7,047 civilians killed. The mission's latest bimonthly report also noted that figures from some governates were not yet included in the total for December. Much of the violence has been blamed on Shiite militias, particularly the Mahdi Army militia that is loyal to radical cleric Muqtada al-Sadr, a key supporter of Prime Minister Nouri al-Maliki. Dozens of bodies turn up on the streets of Baghdad daily, many showing signs of torture. "The root causes of the sectarian violence lie in revenge killings and lack of accountability for past crimes as well as in the growing sense of impunity for on-going human rights violations," the agency said, calling on the Iraqi government to step up efforts to restore law and order. The Iraqi Health Ministry could not immediately be reached for comment, but the government has disputed previous figures released by the UN as "inaccurate and exaggerated." The UN report also said that 30,842 people were detained in the country as of Dec. 31, including 14,534 in detention facilities run by US-led multinational forces. It pointed to killings targeting police, who are seen by insurgents as collaborating with the US effort in Iraq. The report said the Interior Ministry had reported on Dec. 24 that 12,000 police officers had been killed since the war started in 2003. The report also painted a grim picture for other sectors of Iraqi society, saying the violence has disrupted education by forcing schools and universities to close as well as sending professionals fleeing from the country. At least 470,094 people have been forcibly internally displaced since the bombing in Samarra, according to the report, which added that Baghdad alone has 38,766 displaced individuals. The developments came a day after the Iraqi government hanged two of Saddam Hussein's henchmen in an execution that left many of the ousted leader's fellow Sunni Muslims seething after one of the accused was decapitated on the gallows. A thickset Barzan Ibrahim plunged through the trap door and was beheaded by the jerk of the thick beige rope at the end of his fall, in the same the execution chamber where Saddam was hanged a little over two weeks earlier. Dozens of people, mostly schoolchildren, read Quranic verses at the graves in Tikrit as mourning continued for Ibrahim, Saddam's half brother and former intelligence chief, and Awad Hamed al-Bandar, head of Iraq's Revolutionary Court under Saddam. Some 150 youths also staged a demonstration in Saddam's hometown, 130 kilometers (80 miles) north of Baghdad, chanting "down with the pro-Iranian government" and "glory to Barzan," but it was calmer than the day before when at least 3,000 angry Sunnis assembled for the burials. A government video of the hanging, played at a briefing for reporters, showed Ibrahim's body passing the camera in a blur. The body came to rest on its chest while the severed head lay a few yards away, still wearing the black hood pulled on moments before by one of Ibrahim's five masked executioners. At least 13 other people were killed or found dead in Iraq on Tuesday, according to police, including four who died when a roadside bomb struck a police patrol in a predominantly Shiite area of downtown Baghdad.