The Egyptian prime minister has appealed for calm after 24 people were killed in clashes between Coptic Christians and security forces.The violence broke out after a protest in Cairo against an attack on a church in Aswan province last week, which Copts blame on Muslim radicals.PM Essam Sharaf said discord between Muslims and Christians in Egypt was "a threat to the country's security".His comments came after an emergency cabinet meeting.A nighttime curfew was lifted at 07:00 local time (05:00 GMT).Speaking in a televised address after touring the area where the clashes had occurred late on Sunday, Mr Sharaf said: "The most serious threat to the country's security is tampering with national unity, and the stirring of discord between Muslim and Christian sons of Egypt."He added that such violence - the worst in Egypt since President Hosni Mubarak was ousted in February - was also "tampering with the relationship between the people and the army".Earlier, Egyptian TV showed protesters clashing with security forces as army vehicles burned outside the state TV building where protesters had originally planned a sit-in.There were also reports of burning vehicles outside the Coptic hospital, where many of the injured have been taken.Sectarian tensions have increased in recent months in Egypt.The Copts - who make up about 10% of the population - accuse the governing military council of being too lenient on the perpetrators of a string of anti-Christian attacks.Stones hurledThousands - mainly but not exclusively Christians - joined the initial march from the Shubra district of northern Cairo to the state TV building in Maspero Square.They were calling on the military council to sack the governor of Aswan province. They also accused state TV of fanning the flames of anti-Christian agitation.But the demonstrators said they were assaulted by attackers in plain clothes before the clashes with the security forces broke out.The violence began outside the state TV building but soon spread to Tahrir Square, the epicentre of the demonstrations which led to President Mubarak's resignation.Thousands joined in the street violence, tearing up the pavement and hurling stones.Some protesters reported hearing gunfire, and several said they had seen a military vehicle run over at least five people.Eyewitness Nigel Hetherington says troops fired rubber bullets and teargas into crowds. "I saw civilians running past my window as troops fired wildly into the crowds," he told the BBC.The ministry of health said that at least 24 people had been killed and 212 had been wounded in the violence.Of this number, 107 were civilians and 86 were security forces, ministry spokesman Hisham Shiha told the BBC.Simmering tensionsProtesters also called for the resignation of the military council, in particular its chairman, Field Marshal Mohamed Tantawi.The BBC's Yolande Knell in Cairo says sectarian tensions have simmered in the political and security vacuum that has developed in the past couple of months.Christians have been worried by the increasing show of strength by ultra-conservative Islamists.In May, 12 people died in attacks on Coptic churches. In March, 13 people were killed in clashes between Muslims and Copts in Tahrir Square.This latest violence comes ahead of parliamentary elections scheduled for 28 November, the first such vote since former President Hosni Mubarak stepped down.The Copts, the largest minority in Egypt, complain of discrimination, including a law requiring presidential permission for churches to be built. Egypt only recognises conversions from Christianity to Islam, not the other way.