Three people had died in Egypt by early Sunday morning after a day of clashes between police and protesters, amid growing opposition to the ruling military council.
One protester died in Cairo and two others were reported killed in Alexandria.
The Egyptian cabinet planned an emergency meeting on Sunday to discuss the developments, officials said. The government described the violence as "a dangerous issue that directly affects the course of the country and the revolution."
In central Cairo, security forces used tear gas, rubber bullets and shotguns on Saturday to prevent protesters from staging a sit-in against the Supreme Council of the Armed Forces.
By the early hours of Sunday, riot police were fighting thousands of people to force them out of Tahrir square, the scene of the tumultuous demonstrations that drove former president Hosny Mubarak from office.
The Health Ministry said that a 23-year-old man was killed and 750 people were injured. Protesters threw stones at riot police and chanted slogans against the Interior Ministry and the military, activists said.
As clashes in Cairo intensified, thousands took to the streets in other provinces across Egypt.
Witnesses told state-run Al Ahram newspaper that two protesters were killed in the early hours of Sunday in the northern coastal city of Alexandria when snipers fired live ammunition at demonstrators.
Al Ahram reported on its website that police forces were also throwing tear gas canisters at protesters who gathered in front of the Security Directorate of Alexandria.
The Interior Ministry has denied using rubber bullets or live ammunition while trying to disperse the Cairo demonstration, adding that 40 security personnel were among those injured. The statement came after it announced earlier that 20 "rioters" were arrested.
General Mohsen al-Fangary, a member of the Supreme Council of the Armed Forces, which has been ruling the country since the ouster of Mubarak in February, accused Tahrir protesters of seeking to "topple the state" at this critical moment.
"Those in Tahrir Square are not the revolution's injured. Would an injured person be able to throw stones and Molotov cocktails?" he told the Egyptian private Al-Hayat satellite channel.
Witnesses told dpa that protesters searched a man who shot one of the demonstrators in his foot, and found the man's ID showing that he was a member of the police forces.
Protesters have repeatedly accused members of the police forces, loyal to Mubarak's regime, of hiding among the demonstrators to attack them from close range and incite more violence.
Clashes come nine days ahead of parliamentary elections, which many Egyptians fear will be affected by a security vacuum in the country. The three-stage elections are to take place between November 28 and January 10.
The violence erupted Saturday when security forces tried to use force to disperse dozens who have camped out at the iconic Tahrir square, the scene of a mass rally on Friday in which protesters demanded the military rulers quickly transfer power to a civilian government.
The protesters were rallying against what they said was a murky plan from the military to hand over power to a civilian administration.
Thousands of people, most of them Islamists, took part in Friday's rally to demand that military rulers set a clear timetable for the handover.
They also demanded that authorities drop proposed constitutional amendments giving the military powers over the parliament.
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