At least 13 people were killed and several others injured in a clash between two armed groups in Multan district of Pakistan on Wednesday, local reports said.Members of one of the groups opened fire on people belonging to the other when they were coming out of the mosque after offering prayers of Muslim festival of Eid, the Xinhau news agency reported.The firing triggered an exchange of gunfire between the two groups, leaving nine people dead right at the spot and several others injured, a news report said.The injured people were shifted to a nearby hospital where four of them succumbed to injuries.The incident seems to be an outcome of an old rivalry between the groups who are said to be involved in robberies in the area.Police rushed to the incident site following the attack and cordoned it off for investigations.