BAGHDAD — A car bomb explosion tore through a crowd of Shi’ite pilgrims returning Thursday from a religious commemoration, killing at least 20 and reinforcing fears of renewed sectarian violence, according to Iraqi officials.

The blast erupted in the town of Musayyib, about 40 miles south of the Iraqi capital. It targeted worshippers returning from the Shi’ite holy city of Karbala following the climax of the religious commemoration known as Arbaeen. Children were among the 20 people confirmed killed, according to a police official. He said at least 50 people were wounded.