A suicide bomber disguised as a policeman killed at least 53 people and wounded scores in an attack on Shi'ite Muslim pilgrims at a checkpoint on Saturday.
The bombing at the end of Arbain, one of the main religious observances in the Shi'ite calendar, was the worst such incident this year, amid a political crisis and renewed fears of a resurgence of sectarian violence.
A terrorist wearing a police uniform and carrying fake police I.D. managed to reach a police checkpoint and blew himself up among police and pilgrims, said a police official at the scene of the bombing.
The pilgrims had been on their way to a major Shi'ite mosque to the west of Basra, police said.
Security forces sealed off the main hospital in Basra, fearing further attacks as soldiers, police and civilians rushed blood-covered victims to the hospital. Some of the wounded were stuffed into car trunks.
Hundreds of wailing relatives packed into the city morgue searching for casualties. One woman lay on the floor screaming for her dead son and covering her head with dust from the ground in a traditional expression of grief.
Riyadh Abdul-Ameer, director of the Basra health office, told Reuters the blast killed 53 people, with another 130 wounded. Other officials had earlier put the toll at 32 to 35 dead with 90 to more than 100 wounded.
Tensions in Iraq have wider implications in the region. Neighbouring Syria is in turmoil, and Shi'ite Iran and Sunni Arab Gulf nations and Turkey are positioning themselves for more influence.
Arbain marks the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Mohammed who was killed in a 7th century battle in the Iraqi holy city of Kerbala. It has been a repeated target of militants since the U.S.-led invasion that ousted Sunni dictator Saddam Hussein.
Some Sunni Islamist groups such as al Qaeda view Shi'ites as heretics. Saddam banned such Shi'ite religious commemorations.
Hundreds of thousands of Shi'ites have flocked to Kerbala from across Iraq, as well as Iran and other countries in recent days. The observance reached its peak on Saturday.
Scores of people have been killed in attacks on pilgrims in the last few weeks, including a suicide bombing which killed at least 44 people.
Many of the incidents involved methods such as suicide bombings, the signature of Iraq's al Qaeda affiliate.
Shi'ite Prime Minister Nuri al-Maliki's government has issued an arrest warrant for a Sunni vice president, triggering a political crisis that risks scuttling a power-sharing agreement among Sunni, Shi'ite and Kurdish blocs.
Violence had declined since the height of sectarian slaughter in 2006-2007, when thousands were killed in intercommunal strife. But the withdrawal of the last American troops in December has stirred worries of a resurgence.
(Additional reporting by Ahmed Rasheed; Writing by Patrick Markey and Jim Loney; Editing by Andrew Roche)