Foreign Secretary William Hague has urged Iraq's political and religious leaders to "pull together" in the wake of atrocities which have claimed dozens of lives.
At least 69 people were killed in a spate of bombings across the capital Baghdad, just days after US forces withdrew from the country, sparking fears of a spiral of sectarian violence.
Mr Hague branded the attacks "cowardly" and the White House said the attacks served no purpose "other than murder and hatred".
The apparently co-ordinated wave of 16 blasts has heightened tensions in Iraq between Shiite and Sunni communities.
There was no immediate claim of responsibility but the bombings bore all the hallmarks of an attack by mainly-Sunni al Qaida insurgents. Most of the violence appeared to hit Shiite neighbourhoods, although some Sunni areas were also targeted.
In all, 11 neighbourhoods were hit by either car bombs, roadside blasts or 'sticky bombs' attached to cars. At least one of the attacks was a suicide bombing and the explosions went off over several hours.
The worst blast was in the Karrada neighbourhood, where a suicide bomber driving an explosives-laden vehicle blew himself up outside the office of a government agency fighting corruption.
The attacks came during what Mr Hague acknowledged was a time of "increased political tension" in Iraq.
The government of prime minister Nouri al-Maliki, a Shia, has accused Sunni vice president Tariq al-Hashemi of running a hit squad targeting government officials and put out a warrant for his arrest. Mr al-Maliki is also pushing for a vote of no confidence against another Sunni politician, the deputy prime minister Saleh al-Mutlaq.
Mr Hague said: "I condemn the attacks that took place in Baghdad this morning, resulting in the death and injury of a large number of people. I offer my condolences to the bereaved and injured. These cowardly attacks come at a time of political tension in Iraq. I hope that leaders from across the political and sectarian spectrum will pull together to establish a dialogue to ensure Iraq's political stability and to build a stable future. The UK will support Iraq in its efforts to defeat extremism and terrorism."
