At least 15 coordinated explosions in mainly Shia areas of the Iraqi capital have left scores dead and dozens wounded Reuters

Scores of people have been killed and hundreds injured in a series of bombings targeting mainly Shia neighbourhoods in Baghdad.

At least 15 blasts, many of them caused by car bombs, prompted fears the violence heralded the second phase of an al-Qaida attempt to incite sectarian chaos after a massacre inside a church on Sunday.

The authorities are scrambling to deal with the aftermath of the co-ordinated bombings.

At least 76 people were reported killed and nearly 200 injured, but the death toll appears certain to rise.

The most deadly incident took place in Sadr City, where 21 people were killed. Hussein al-Saiedi, a resident of the Shia slum, said: "They murdered us today, and on Sunday they killed our brothers the Christians. We were just standing on the street when we heard a loud noise, and then saw smoke and pieces of cars, falling from the sky."

Security forces have ordered shopkeepers to return home and have closed down main roads in the east Baghdad, which is mainly Shia. Apart from election days, Baghdad has not seen a city-wide curfew since late 2007.

The bombings came hours after a memorial service for some of the 52 hostages and security officers killed on Sunday, when Islamist gunmen stormed the Our Lady of Salvation church. Since then, security chiefs and politicians had ordered extra protection around mosques and churches, fearing further attacks. Police and soldiers used loudspeakers to order residents to stay in their homes near known sectarian flashpoints.

Tonight's bombs detonated within 90 minutes of each other. Hospitals were appealing for blood donors, and the city's main A&E centres were reporting large numbers of casualties amid chaotic scenes. The bombs exploded in 12 areas of the city, including a police station in Sadr City and a coffee shop in New Baghdad. Restaurants appeared to be prominent targets in other attacks, along with main roads and, in one case, a funeral tent.

The scale of the attacks, and the ease with which car bombs were again able to penetrate security cordons, constitute a damaging blow for Iraq's security forces, which have remained without effective leadership for eight months owing to the crippling political crisis that has seen politicians unable to form a government. Residents have dreaded a slide towards sectarian violence. Many have used the relative lack of large-scale violence this year to reopen businesses and re-establish lives put on hold by eight years of war.

Car bombings had been rare since January, apart from several days of widespread nationwide violence in April and late August. Security chiefs immediately blamed al-Qaida for the latest attacks. Al-Qaida in Iraq is an umbrella group for many Sunni Islamist organisations who align to a global jihadi worldview that identifies Shia Muslims and Christians as mortal foes.

An intelligence chief told the Guardian that al-Qaida in Iraq had regrouped this year after briefly aligning during 2008 with former leaders of Saddam Hussein's Ba'athist regime. "This is pure al-Qaida," he said. "It's back to like it was in the early days – around 2004."

He said the group had re-established training camps in Iraq, some as discreet gatherings but others, in areas in rural areas, more openly. "There are many places: farms, deserts, private residences," he said. "Iraq is a big place, and it is easy to hide."

The head of the Shia Endowment group, the main body for Shia interests in Iraq, said: "The goal is to reignite sectarian violence in Iraq. We have a responsibility to calm this demand and clarify things."

Denouncing the attackers as apostates, he said: "They want a civil war and they want to stop the political process. As soon as we see a ray of hope here, these terrorists rise again."