After 18 days, the demands of tens of thousands of anti-government demonstrators were met as Egyptian Vice President Omar Suleiman announces that Mubarak has stepped down and handed over power to the military. Nearly 300 people have been killed since anti-government protests started up in Egypt three weeks ago.
The Internet and cell-phone data service appeared to be cut across Egypt on Friday as authorities braced for demonstrations backed by both the country's biggest opposition group and newly returned Nobel Peace laureate Mohamed ElBaradei.
The government deployed an elite special operations force in Cairo on Thursday night as violence escalated outside the capital, and the Islamist Muslim Brotherhood called on its members to take to the streets after Friday afternoon prayers.
Uniformed security forces at least temporarily disappeared from the streets of central Cairo mid-morning Friday, but truckloads of riot police and armored cars started moving back about an hour later.
Unconfirmed reports circulated early Friday on Twitter that police were splashing gas around key squares ready to set them alight when protesters approached.
The Muslim Brotherhood said at least five of its leaders and five former members of parliament had been arrested.
The group's lawyer, Abdel-Moneim Abdel-Maksoud, and spokesman, Walid Shalaby, said a large number of rank-and-file Brotherhood members also had been detained.
Egypt's four primary Internet providers -- Link Egypt, Vodafone/Raya, Telecom Egypt, Etisalat Misr -- all stopped moving data in and out of the country at 12:34 a.m., according to a network security firm monitoring the traffic. Telecom experts said Egyptian authorities could have engineered the cutoff with a simple change to the instructions for the companies' networking equipment.
The Internet appeared to remain cut off Friday morning, and cell-phone text and Blackberry Messenger services were all cut or operating sporadically in what appeared to be a move by authorities to disrupt the organization of demonstrations.
Egyptians outside the country were posting updates on Twitter after getting information in voice calls from people inside the country. Many urged their friends to keep up the flow of information over the phones.
The developments were a sign that President Hosni Mubarak's regime is toughening its crackdown following the biggest protests in years against his nearly 30-year rule.
The counter-terror force, rarely seen on the streets, took up positions in strategic locations, including central Tahrir Square, site of the biggest demonstrations this week.
The real test for the protest movement will be whether Egypt's fragmented opposition can come together, with Friday's rallies expected to be some of the biggest so far.
The movement's momentum appeared to gather Thursday with the return of Nobel Peace laureate Mohamed ElBaradei and the backing of the Muslim Brotherhood.
Social networking sites were abuzz that the gatherings called after Friday prayers could attract huge numbers of protesters demanding the ouster of Mubarak. Millions gather at mosques across the city on Fridays, giving organizers a vast pool of people to tap into.
The 82-year-old Mubarak has not been seen in public or heard from since the protests began Tuesday with tens of thousands marching in Cairo and a string of other cities. While he may still have a chance to ride out this latest challenge, his choices are limited, and all are likely to lead to a loosening of his grip on power.
Violence escalated on Thursday at protests outside the capital. In the flashpoint city of Suez, along the strategic Suez Canal, protesters torched a fire station and looted weapons that they then turned on police. The Interior Ministry said in a statement that more than 90 police officers were injured in those clashes. There were no immediate figures on the number of injured protesters.
In the northern Sinai area of Sheik Zuweid, several hundred Bedouins and police exchanged gunfire, killing a 17-year-old. About 300 protesters surrounded a police station from rooftops of nearby buildings and fired two rocket-propelled grenades at it, damaging the walls.
The United States, Mubarak's main Western backer, has been publicly counseling reform and an end to the use of violence against protesters, signs the Egyptian leader may no longer be enjoying Washington's full backing.
In an interview broadcast live on YouTube, President Barack Obama said the anti-government protests filling the streets show the frustrations of Egypt's citizens. "It is very important that people have mechanisms in order to express their grievances," Obama said.
On its website, the Muslim Brotherhood said it would join "with all the national Egyptian forces, the Egyptian people, so that this coming Friday will be the general day of rage for the Egyptian nation."
The Brotherhood has sought to depict itself as a force pushing for democratic change in Egypt's authoritarian system, and is trying to shed an image among critics that it aims to seize power and impose Islamic law. The group was involved in political violence for decades until it renounced violence in the 1970s.
ElBaradei, the former head of the U.N. nuclear watchdog and a leading Mubarak opponent, has sought to recreate himself as a pro-democracy campaigner in his homeland. He is viewed by some supporters as a figure capable of uniting the country's fractious opposition and providing the movement with a road map for the future.
Speaking to reporters Thursday before his departure for Cairo, ElBaradei said: "If people, in particular young people, ... want me to lead the transition, I will not let them down. My priority right now ... is to see a new regime and to see a new Egypt through peaceful transition."
Once on Egyptian soil, he struck a conciliatory note.
"We're still reaching out to the regime to work with them for the process of change. Every Egyptian doesn't want to see the country going into violence," he said.
With Mubarak out of sight, the ruling National Democratic Party said Thursday it was ready for a dialogue with the public but offered no concessions to address demands for a solution to rampant poverty, unemployment and political change.
Its comments were likely to reinforce the belief held by many protesters that Mubarak's regime is incapable, or unwilling, to introduce reforms that will meet their demands. That could give opposition parties an opening to win popular support if they close ranks and promise changes sought by the youths at the forefront of the unrest.
Mubarak has not said yet whether he will stand for another six-year term as president in elections this year. He has never appointed a deputy and is thought to be grooming his son Gamal to succeed him despite popular opposition. According to leaked U.S. memos, hereditary succession also does not meet with the approval of the powerful military.
Mubarak has seen to it that no viable alternative to him has been allowed to emerge. Constitutional amendments adopted in 2005 by the NDP-dominated parliament has made it virtually impossible for independents like ElBaradei to run for president.
The Associated Press contributed to this report.