Police in Cairo have used tear gas and water cannon to disperse protesters camped out in the center of the Egyptian capital.
The show of force came overnight after thousands of demonstrators took to the streets in several Egyptian cities in unprecedented protests that turned deadly as crowds demanded political and economic reforms.
At least two protesters and one police officer were reported to have been killed in the violence on January 25.
Organizers of the demonstrations -- in the capital, Cairo, as well as in Alexandria -- have been inspired in part by Tunisia's recent uprising
Police fired tear gas at protesters in central Cairo on January 2as the demonstrators broke through barricades and demanded the ouster of President Hosni Mubarak.
The scene of demonstrators marching on Cairo's central Tahrir Square and along the Nile River was a public display of antigovernment discontent that has rarely been seen on the streets there during Mubarak's three decades of rule.
Some in the crowd chanted, "Get out, Mubarak!"
Riot police had been called out in force after as many as 80,000 Egyptians vowed on Facebook and other new-media social networks to join a nationwide antigovernment protest.
Crowds swelled and got increasingly unruly. A large security force patrolled the streets of Cairo, arresting people, and chasing others into side streets.
Online activists in Egypt, the Middle East's most populous country, have become some of the most vocal critics of Mubarak's rule.
Twitter, the Internet messaging service used by demonstrators to organize, said it had been blocked in Egypt. Eyes On Tunisia?
"I think Egyptian young [people] mainly have been inspired by what happened in Tunisia, and they saw one of the most prominent dictatorships in the Arab region -- our neighbors -- leave [due to] the people's pressure," Ehab el-Zelaky, managing editor of the Cairo-based "Al-Masry Al-Youm" newspaper, told RFE/RL's Radio Farda. "And I think many of them are saying, 'Why not? Why can't we [make] our own destiny here in Egypt also?'"
The January 25 show of dissent -- which coincided with a holiday honoring Egypt's police -- was first announced by a pro-democracy youth group called "The April 6 Movement." It was organized and promoted online through social networks like Facebook and Twitter as "Revolution Day Against Torture, Poverty, Corruption and Unemployment."
The Muslim Brotherhood, Egypt's largest opposition movement, did not formally endorse the protests but said many of its members would take part as individuals.
In Cairo, as many as 30,000 police mobilized in the city center as protesters outside the Supreme Court building held large signs that read, "Tunisia is the solution."
Security officials said the Interior Ministry was "locked down" after crowds began to gather in a square in front of the ministry building.
Protests in Egypt tend to be poorly attended and often are quashed swiftly by police who prevent demonstrators from marching. But the Cairo protest, and other demonstrations of discontent in Alexandria and elsewhere, threatened to spiral out of authorities' otherwise tight grip.
Political analysts have been closely watching the latest events as a test of whether online activists in Egypt could translate their messages into a massive political street action. The protests also were being watched as a sign of what impact the Tunisian revolt has had on the mobilization of opposition movements elsewhere in the Arab world.
Some two out of three Egyptians now have access to mobile telephones. That has raised concerns within Egypt's security forces that social networking could be used by protest organizers to quickly mobilize and regroup crowds that may be dispersed by security forces.
Interior Minister Habib el-Adli issued orders to "arrest any persons expressing their views illegally." Egypt bans protests without prior permission, and protest organizers said they'd been denied permits for the January 25 rallies.Opposition Disunity
Egypt's registered opposition political parties are seen as being weak and fragmented. The failure of the banned Muslim Brotherhood to join calls for action has frustrated some of Egypt's younger opposition activists who appear to be growing tired of the group's older generation. That is making today's events a kind of generational test of government opposition in Egypt as well.
In an interview with the state's "Al-Ahram" newspaper, Adli suggested the demonstrations -- like the initial protests in Tunisia -- would be attended primarily by younger opposition activists.
Adli was quoted by that newspaper as saying: "Youth street action has no impact and security is capable of deterring any acts outside the law."
The human rights watchdog Amnesty International has urged Egyptian authorities "to allow peaceful protests."
U.S. Secretary of State Hillary Clinton told reporters in Washington that "We support the fundamental right of expression and assembly for all people and we urge that all parties exercise restraint and refrain from violence."
Clinton added: "Our assessment is that the Egyptian government is stable and is looking for ways to respond to the legitimate needs and interests of the Egyptian people."
Egypt is one of Washington's staunchest allies in the Middle East and a major recipient of U.S. aid.based on RFE/RL and agency reports