Twenty three people were killed in Cairo on Sunday, the health ministry said, when Christians, some carrying crosses and pictures of Jesus, clashed with military police in the latest sectarian flare-up in the country.

Christians protesting against an attack on a church say they were marching peacefully when thugs attacked them, drawing in military police who used what activists described as unnecessary force.

More than four vehicles were set ablaze and television footage showed army personnel carriers driving full speed towards crowds of protesters.

Pictures and videos of smashed faces and dead bodies of what activists said were people run over by military vehicles circulated online, prompting angry comments comparing the violence used by the military to that of ousted President Hosni Mubarak's police during the uprising that toppled him this year.

"What happened today is unprecedented. Seventeen corpses were crushed by military trucks," human rights activist Hossam Bahgat said from the hospital where the bodies were taken.

Protesters threw rocks and petrol bombs and set cars on fire, as thick smoke wafted through the streets in some of the most violent scenes since the uprising that ousted Mubarak.

Hundreds fought with sticks on a Cairo bridge and protests later spread to Tahrir Square, the focal point of the February uprising, as Muslims joined the rally out of solidarity.

Smoke from tear gas swirled over the square as thousands of protesters chanted "The people demand the fall of the field marshal", referring to Mohamed Hussein Tantawi, head of Egypt's army council which now rules the country.

A Reuters witness said the army had moved in to contain the unrest, beating some protesters with batons. State television announced a curfew from 2 a.m. to 7 a.m to apply to Tahrir and downtown Cairo, as well as any roads leading to the square.

State TV said at least 150 people were injured on Sunday, without saying how many of them were protesters. It had earlier said three of those killed were soldiers.

Tensions between Christians and Muslims have increased since the uprising. The latest violence comes just weeks before a parliamentary election on Nov. 28, the first such vote since Mubarak was ousted.

The government has appealed for calm and denied that the violence was sectarian in nature. Prime Minister Essam Sharaf said he had contacted security and church authorities to contain the situation.

"The only beneficiary of these events and acts of violence are the enemies of the January revolution and the enemies of the Egyptian people, both Muslim and Christian," he said on his Facebook page.

"What happened is not sectarian strife. We should be aware of this. This is a conspiracy to delay the elections," Sharaf told state television late on Sunday.

"We will apply the law with all strictness against anyone who seeks to set fire to this nation."

Tantawi called on the Prime Minister to hold an emergency meeting. The cabinet also said in a statement that it would not allow Sunday's events to undermine the country's transition.

Christians, who make up 10 percent of Egypt's roughly 80 million people, took to the streets after blaming Muslim radicals for partially demolishing a church in Aswan province last week. They also demanded the sacking of the province's governor for failing to protect the building.  - Reuters