Egypt's military rulers ordered a speedy probe into clashes which killed 25 people, mostly Coptic Christians, as the cabinet held crisis talks amid fears of wider sectarian unrest and Washington urged restraint.
The Supreme Council of the Armed Forces (SCAF) "tasked the government with quickly forming a fact finding committee to determine what happened," in a statement read on state television.
It called for "all measures against all those proven to have been involved, either directly or by incitement."
Both the ruling military council and the cabinet held crisis talks, a day after the clashes in Cairo left 25 dead, most of them Copts, and more than 300 people injured, according to health ministry figures.
SCAF, which took power when president Hosni Mubarak was ousted in February, stressed it "continues to bear national responsibility to protect the people" until it hands over to an elected civilian authority.
US President Barack Obama is "deeply concerned" about the violence, the White House said.
"Now is a time for restraint on all sides so that Egyptians can move forward together to forge a strong and united Egypt," spokesman Jay Carney said.
"As the Egyptian people shape their future, the United States continues to believe that the rights of minorities - including Copts - must be respected, and that all people have the universal rights of peaceful protest and religious freedom.
"These tragic events should not stand in the way of timely elections and a continued transition to democracy that is peaceful, just and inclusive."
Military prosecutors began questioning 25 people accused of involvement in the clashes, state news agency MENA reported, after a security official said 40 people were arrested overnight.
Copts had been protesting against an attack earlier this month on a church in the southern city of Aswan when the violence erupted. Fighting later near the hospital caused more casualties.
Egypt's Coptic Orthodox church led by Pope Shenuda III accused "infiltrators" of triggering the street battle on the Nile waterfront.
"The Christian faith denounces violence. Strangers infiltrated the demonstration and committed the crimes for which the Copts have been blamed," it said after Shenuda met 70 church leaders.
"Copts have suffered repeated problems without accountability for the aggressors," it said in a statement, calling on authorities to "solve the root causes of the problems."
The clashes rattled the Cairo stock exchange, which shed 5.15 per cent within minutes of opening.
European leaders gathered for talks in Luxembourg expressed alarm at the Cairo clashes.
A curfew was imposed overnight in parts of the Egyptian capital.
In a late-night address, Prime Minister Essam Sharaf appealed to Egyptians "not to give in to sedition" and warned the country was "in danger."
At least five of the dead were mown down by a speeding army vehicle which swerved to hit protesters, a Coptic priest said, while an AFP correspondent saw bodies with gunshot wounds at the hospital.
The grand imam of Al-Azhar, Ahmed Tayyeb, the Sunni religious head, called for crisis talks between Muslim and Christian leaders "in a bid to contain the crisis," state television said.
Some commentators warned of civil religious strife, but others said the clashes were fuelled not only by sectarian strife but also by anger towards the security services and the military council which succeeded Mubarak.
Government-backed provocation was also blamed.
Users of social networking sites such as Twitter said the initial clashes outside the state television building on the Nile were provoked by "thugs" at the scene, while the broadcaster was accused of fanning anti-Coptic sentiment.
On Monday, the authorities announced the hanging of a man sentenced to death for shooting six Copts last year.
The protesters in Cairo clashed with anti-riot police and soldiers guarding the state television building, after thousands staged a protest march from the Shobra district.
A standoff degenerated as the demonstrators hurled stones and set fire to two cars, an AFP correspondent said. An army vehicle was also burned.
"Down with the marshal," the thousands of demonstrators chanted, referring to SCAF chief Field Marshal Hussein Tantawi.
State television reported that three soldiers were shot dead and dozens wounded.
Copts complain of systematic discrimination, but since Mubarak's fall, tensions have also mounted between the military - initially hailed for not siding with Mubarak - and groups which spearheaded the revolt.