Bombers and gunmen killed 40 people in attacks on Shia worshippers marking the religious Ashura ritual today, amid heightened tension between Iraq's majority Shia Muslim community and once-dominant Sunni Arabs.
Sunnis too came under attack - from mortar bombs that rained down on the Adhamiya district in Baghdad, killing 17, a police source said. A hospital doctor put the death toll at 20.
The deaths on the final day of the annual week-long Ashura ritual, highpoint of the Shia religious calendar, underlined widespread concerns about Iraq sliding into sectarian civil war.
Tens of thousands of Iraqis have been killed in sectarian bloodshed between Shia and Sunnis since an attack on a Shia mosque in Samarra in February 2006.
Security was tight for fear of a repetition of suicide bombings and other attacks on Ashura crowds of the sort that killed 171 people in Baghdad and the holy city of Kerbala in March 2004. This time Kerbala escaped incident, and the bombers appeared to have focused on other less protected Iraqi towns.
Fearing a possible strike by Sunni insurgents, Iraqi authorities had deployed 11,000 police and soldiers to Kerbala, focus of the commemoration that marks the death in battle of the Prophet Mohammed's grandson 1,300 years ago.
Ashura has been a target in the past for radical Sunni militants who view the Shia community - a majority in Iraq but a minority in the Islamic world - as heretics.
The fears were fuelled by the discovery of what Iraqi officials said was a plot by a messianic Muslim cult to target senior Shia clerics in the holy city of Najaf south of Baghdad at the climax of Ashura this week.
Iraqi forces backed by US tanks and warplanes fought a day-long battle with the so-called "Soldiers of Heaven" near Najaf on Sunday. Officials said the cult's leader was killed.
Iraqi Defence Ministry spokesman Mohammed al-Askari said on Tuesday the final toll from the battle was 263 killed. A total of 502 "Soldiers of Heaven" followers had been arrested, including 210 wounded.