At least 21 people were killed Sunday when clashes erupted between mostly Coptic Christian demonstrators and army forces outside the state television building in central Cairo, dpa quoted medical sources as reporting.
The deaths included 17 demonstrators and four soldiers with more than 150 people injured in the clashes, said the sources.
The demonstrators, who were protesting the destruction of a church in southern Egypt, torched two armoured vehicles, six private cars and a public bus, security sources said. Scores of suspected assailants were arrested in the aftermath, the security sources said.
"More than 10 Copts died," said Rami Kamel, who led the protest. "Anonymous people shot on the demonstrators."
State television reported that the demonstrators threw stones and fired on soldiers who were guarding the Nile-side TV and radio building.
Activists wrote online that several civilians attacked the protesters and the army forces.
State television and radio employees were trapped Sunday evening inside the building, after the army closed all doors and prevented anyone from leaving, witnesses told dpa. Several roads around the area were closed by protesters, who prevented cars from passing, they said.
Prime Minister Essam Sharaf said that the clashes were an attempt to create chaos in Egypt and ignite sectarian tension.
In a statement on his Facebook page, Sharaf urged Egyptians to remain united against extremist "vandalizing forces."
"The enforcement of law on everyone is the best solution to all of Egypt's problems," Sharaf wrote.
Sharaf is slated to lead a Monday mini-cabinet meeting, with Muslim and Coptic leaders in attendance to discuss the incident, state television reported.
Hundreds of mostly Christian demonstrators have been protesting for several days over the destruction of a church on September 30 in the southern Egyptian province of Aswan.
During Sunday's protest, led by several bishops, the demonstrators burnt photos of Aswan Governor Mustafa al-Sayed, who had said that the church in the village of Marinap had been built illegally. The protesters demanded that al-Sayed be sacked and the church rebuilt.
Earlier Sunday in the southern city of Luxor, a group of protesters briefly blocked the city's main roads. The governor convinced them to leave after promising to resolve the issue of the destroyed church.
Christians account for around 10 per cent of Egypt's 80 million people, and tensions are not uncommon with the country's Muslim majority.
In March, 13 people were killed in sectarian clashes around the Cairo neighbourhood of Manshiyet Nasser, shortly after a church was torched in the village of Sol, south of the capital.