LAHORE, Pakistan — At least 55 people were killed and at least 120 more were wounded Sunday evening when a suicide bomber set off explosives at a border post in eastern Pakistan, police officials said.

The deadly explosion occurred in Wagah, a town on the border with India and on the outskirts of Lahore, the provincial capital of Punjab Province, where a flag-lowering ceremony takes place daily before sunset.

The spectacle attracts thousands on both sides of the border as people cheer and applaud smartly dressed contingents of the Pakistani Rangers and the Indian Border Security Force.

The bomber, believed to have been in his early 20s, detonated a suicide vest as spectators, including women and children, were leaving after the ceremony, said Mushtaq Sukhaira, the inspector general of the Punjab police.