Iraqi authorities say a car bomb has killed at least 20 Shi'ite Muslim pilgrims on their way home from a religious procession in the country's south.
Police said Thursday's attack at a busy bus station in the town of Musayyib hit the pilgrims as they were returning from Iraq's shrine city of Karbala, where they performed mourning rituals for a revered figure in Shi'ite Islam. Dozens were wounded in the bombing.
No one has claimed responsibility for the attack, which has renewed fears of an increase in sectarian violence that could further destabilize the country.
The blast comes just days after a wave of bombings across the country killed 23 people. Insurgents blew up several houses in the town of Musayyib on Monday, killing seven people. Bombings also killed people in Baghdad, Hillah and Kirkuk.
Minority Sunni militants have targeted majority Shi'ite pilgrims frequently. Those militants also appear to be exploiting sectarian tensions in the ruling coalition of Shi'ite Prime Minister Nouri al-Maliki, who is under political fire.
Sunni protesters have held more than a week of anti-government protests in the western province of Anbar, demanding an end to what they see as Maliki's marginalization of their community and its representatives in his Shi'ite-led coalition.
Shi'ite religious observation
Arbaeen marks the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Mohammad and a central figure of Shi'ite Islam who was killed in a seventh century battle. His followers believe he was buried in Karbala.
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Pilgrims attending the ceremony in the golden temple of the Imam Hussein beat their heads and chests whilst singing.
Snipers and bomb squads were among around 30,000 Iraqi police and soldiers deployed around Karbala to watch over millions of pilgrims gathered to observe Arbaeen.
Shi'ites have been travelling by foot to observe the annual rite which is a major test for Iraqi security forces after a series of attacks targeted Shi'ite pilgrims across Iraq.
Shi'ite religious rites were banned under the rule of Saddam Hussein, who was ousted in 2003 by a U.S.-led invasion that triggered years of sectarian violence.
Explosions across Iraq killed at least 23 people and wounded 87 on Monday, police said, amid a growing political crisis that is inflaming sectarian tensions.
The pilgrimage site has been a repeated target of militants since the U.S.-led invasion that ousted Sunni dictator Saddam Hussein.
Last year, a suicide bomber disguised as a policeman killed at least 53 people and wounded scores in an attack on pilgrims at the end of Arbaeen.
Hussein, a Sunni, placed strict limits on pilgrimages to Kerbala, but since his overthrow in 2003, these have become a show of strength for Iraq's Shi'ite majority and a prime target of Sunni Islamist insurgents.
Meanwhile Iraq's Shi'ite prime minister, Nuri al-Maliki, on Monday warned he will not tolerate Sunni anti-government rallies indefinitely, but made a concession to their demands by promising to free some women prisoners.
Thousands of Sunnis have been taking to the streets of Iraq for more than a week in protest against Maliki, whom they accuse of discriminating against their sect and being under the sway of their non-Arab Shi'ite neighbor Iran.
The incident has once more threatened to plunge a delicate power-sharing deal into turmoil, just as President Jalal Talabani, a moderating influence, is in Germany for medical care after suffering a stroke.
Some information for this report was provided by AP, AFP and Reuters.