A suicide bomber killed 12 people in southern Afghanistan on Thursday, while the police opened fire at rock-throwing protesters, enraged at the civilian deaths that they allege were caused by American bombing. The suicide bomber, riding a motorbike, set off explosives near a coalition forces vehicle in the Gereshk district market area in the Helmand province. The explosion killed 12 people and wounded at least 32, said Daud Ahmadi, spokesman for the Gereshk governor.