CAIRO - Egypt's Coptic Pope Shenouda III has appealed for calm after a New Year's Day bombing killed 21 churchgoers in the coastal city of Alexandria. The spiritual head of the Middle East's largest Christian minority has also called on the Government to address his flock's grievances. The unrest came as police went on high alert and tightened church security for Christmas, which Copts celebrate on Friday, as investigators hunted the perpetrators of the Alexandria church bombing. Tensions spilled over again late on Monday as protesters in a northern Cairo neighbourhood threw rocks at police who tried to block a march by thousands of Copts. Riot police responded by throwing rocks back at the protesters, who also pelted armoured police cars with stones and bottles. "I call on our sons for calm, as calm can solve all issues," the Coptic leader said in a television interview, the text of which was also published by the official Middle East News Agency (MENA). "The slogans used by some have transgressed all values and manners... and some have tried to use violence, while violence is absolutely not our method," he said, blaming unspecified people he said were unrelated to his community. Protesters also blocked a ring road in two districts of the Nile Delta province of Qalubiya, burning tyres and stoning cars, MENA reported. In the interview, Shenouda said the State must address Coptic complaints. "If it finds a law that harms some, the State has to correct the law or legislate new ones," he said. Coptic Christmas this year falls on the weekly Muslim day of prayer and rest, and Shenouda said he intended to say Mass as usual on Christmas Eve. Egyptian security forces cancelled leave for senior officers and tightened surveillance of airports and ports to prevent suspects from leaving the country, as new checkpoints were set up across the nation.