At least 12 people were injured by a grenade explosion outside a Catholic church in the southern Philippines during a Christmas Eve mass on Saturday, police said.The grenade was lobbed at a police patrol car parked in front of the Sto. Nino Parish Church, about 1,600 kilometres south of Manila, police Superintendent Romeo Galgo Junior said.The victims were churchgoers who were standing outside the parish because it was packed with people, Galgo said."The mass was abruptly ended, there were no more concluding prayers as there was a commotion," he said. "People hurriedly left the church."Authorities were still determining a motive for the attack.In the past, local terrorist groups with links to the international network al-Qaeda have bombed Catholic churches during the Christmas holidays in the Philippines.Christmas is an important holiday in the Philippines, where more than 80 per cent of the population is Roman Catholic.