At least 22 people have been killed and many injured in a suspected suicide bomb that struck a religious procession in north-west Pakistan. The explosion tore through a crowd of Shia Muslims marking the festival of Ashura in the town of Hangu. No-one has claimed responsibility for the attack. Pakistan has a history of tension between Shia and Sunni Muslims. Troops were sent to maintain a curfew after pilgrims responded to the attack by torching shops and cars.The explosion erupted in a bazaar as hundreds of people walked in a procession from the main Shia mosque in the town, in North-West Frontier Province. "We thought the bomb was detonated by remote control, but now it appears to be a suicide attack," local police chief Ayub Khan told the Associated Press news agency. One report said the devotees were also sprayed with gunfire. Maulana Khurshid Anwar, a leader of the Shia procession, told AP the explosion happened just as he was about to address the crowd. District administrator Ghani ur-Rehman said the ensuing violence destroyed 60% of the town's bazaar. A judicial inquiry into the attack has been ordered, officials said.