Baghdad, Iraq (CNN) -- A suicide bomber disguised as a policeman targeted Shiite pilgrims Saturday outside the southern Iraqi city of Basra, killing at least 53 people and wounding 137, the Interior Ministry said.
The blast was the single deadliest attack on the Shiite community since the U.S. military completed its troops withdrawal from Iraq. A January 5 suicide attack on Shiite pilgrims west of the southern city of Nasiriya killed 44.
Saturday's bomber struck a police checkpoint near a Shiite mosque where many of the pilgrims were headed to mark a revered holy day, the end of a 40-day mourning period known as Arbaeen, Interior Ministry officials said.
President Jalal Talabani called on security forces to beef up their efforts in protecting the war-torn country.
"In a new attempt to incite sectarian sedition and drive the country to the brink of civil war and to terrorize the people , the terrorist forces have committed a series of heinous crimes that have targeted Imam Hussain pilgrims," Talabani said.
The U.S. Embassy in Baghdad issued a statement condemning the attack, saying, "The United States Government and the American people stand firmly in defense of the right to practice one's faith and condemn any attempts to deny people this basic right. Such senseless acts of violence tear at the fabric of Iraqi unity."
Martin Kobler, the U.N. special representative in Iraq, also condemned the bombing.
Scores of pilgrims -- who make the journey on foot -- have been killed and wounded in recent days in attacks blamed on Sunni extremists, who have routinely targeted the pilgrims.
Arbaeen, considered one of the holiest days in the Shiite sect, commemorates the seventh century death of Imam Hussein, the grandson of the Prophet Mohammed.
Hussein was killed during a battle near Karbala for control of the Muslim nation following Mohammed's death in 632. Hussein's death led to a split between Sunni and Shiite Muslims.
During Saddam Hussein's rule, the Sunni leader forbade pilgrims from making the journey on foot. He also banned some ritualistic practices.
Following the U.S.-led invasion that toppled Saddam, the pilgrimages grew in numbers.
This year's pilgrimage comes just weeks after the U.S. military pulled its troops from the country ahead of an end-of-the-year deadline to withdraw.