The protests also spread to the country’s second-largest city, Alexandria, where residents said demonstrators gathered outside the main military command center and blocked several streets.
Sunday’s violence was the clearest signal yet that Egyptians are turning on the military commanders who were widely hailed as saviors eight months ago, when President Hosni Mubarak was forced from power. Many Egyptians say that swelling anger over the slow pace of reforms and the country’s economic woes could give rise to a new revolt.
“Down, down, Field Marshal Tantawi!” protesters chanted, referring to the leader of the ruling military council.
In an apparent move to discourage a new sit-in in Tahrir Square, the cabinet said late Sunday that it would impose a curfew in the area from 2 to 7 a.m.
The clashes unfolded amid growing criticism of the military leaders, who have continued to rely on loathed policies from the Mubarak era. Many Egyptians reacted with dismay to the recently announced electoral timeline, which would put off presidential elections until 2013.
“What happened today will definitely increase people’s anger toward the military and the anger of the entire Christian population,” said Wael Abbas, a well-known activist and blogger. “People’s rights are being violated, and nothing real is happening.”
Earlier Sunday, the ruling council said it would no longer try civilians in military courts, apparently bowing to pressure from activists.
Coptic Christians have been among the most vocal critics of the council. They complain that the military leadership has done too little to protect them in the wake of a string of attacks on churches this year. Coptic Christians, who make up about 10 percent of the country’s 80 million people, blame fundamentalist Muslims for the violence.
Sunday’s march began in the predominantly Christian district of Shubra, in northern Cairo, as demonstrators took to the streets to condemn the recent attack on a church in southern Egypt. The marchers were headed downtown toward the state television building, which in recent months has become a backdrop for sit-ins and demonstrations.
When they were a few blocks away, men in civilian clothes attacked the protesters with rocks, witnesses said.
Demonstrators said a harrowing and confusing scene followed. As reports of gunfire and rock-throwing spread through word of mouth and social media, thousands, including Muslims who joined the Coptic marchers, swarmed toward the state television building, where intense clashes with riot police broke out.
Security and civilian vehicles drove erratically through the area, mowing people down, witnesses said.
Mourkos Aziz, 32, a demonstrator, said he saw a police personnel carrier slam into a crowd, killing at least six people.
“They fired on us,” protester Morkos George, 30, said outside the television building, referring to security forces. “We had no weapons.”
Demonstrators retaliated by setting police vehicles on fire using molotov cocktails — a tactic widely used during the early days of the uprising to strike at the symbol of a police state Egyptians had feared for decades.
State television reported that three soldiers were among the 23 people killed in the clashes. The channel said Coptic protesters instigated the clashes, but there appeared to be little evidence to back up that claim. Human rights activist Hossam Bahgat said on Twitter that he saw 17 bodies at the Coptic Christian hospital morgue — which suggested that most of the dead were Christian. Witnesses said the death toll was likely to be higher.
Shortly after the clashes began, Egyptian state television and radio broadcast a message urging “honest Egyptians” to protect the military. Egyptian activists called the message inflammatory, saying it probably fueled the violence.
As fighting intensified, witnesses said, the violence appeared to be fueled by religious fervor. Men wielding clubs raced toward Tahrir Square shouting “no God but Allah,” and “the people want to overthrow the Copts,” said Elijah Zarwan, a Cairo-based analyst who witnessed the scene.
“What I saw and heard today is a side of Egypt that I’ve never seen before,” said Zarwan, who works for the International Crisis Group, a think tank. “There have always been sectarian tensions simmering under the surface, but now something very dangerous has been unleashed.”
Military police barged into the Cairo studio of the U.S.-funded Alhurra satellite television channel and the independent TV 25 news channel to stop them from broadcasting live footage of the mayhem nearby, according to Egyptian news site al-Ahram Online.
Egyptian Prime Minister Essam Sharaf called for restraint and urged Egyptians not to “give into strife.” He convened an emergency cabinet meeting late Sunday.
“The cabinet’s crisis management committee is meeting now to discuss the developments of the unfortunate events the country is witnessing,” said a statement posted on the cabinet’s Facebook page Sunday night.
Special correspondent Ingy Hassieb contributed to this report.