In the last week, al-Jazeera's bureau was forcibly closed, its journalists' press credentials were revoked and nine journalists were detained, the network said in a statement. The network has also faced unprecedented levels of interference in its broadcast signal, as well as persistent and repeated attempts to bring down its Web sites.
"We are grateful for the support we have received from across the world for our coverage in Egypt," al-Jazeera said, "and can assure everyone that we will continue our work undeterred."
The army presence outside Tahrir Square seemed substantially bigger Friday than it had been earlier in the week, with soldiers maintaining a tight security perimeter and patrolling on rooftops of nearby buildings.
In the early afternoon, al-Jazeera reported that Arab League secretary general Amr Moussa, a former foreign minister of Egypt, had entered the square. Moussa is seen by many as a more likely replacement for Mubarak than Nobel laureate Mohamed ElBaradei - the favorite of many demonstrators - because he hasn't been outside the country for most of his career.
Moussa told France's Europe 1 radio that he was "at the disposal of my country. . . ready to serve as a citizen who has the right to be a candidate" for president.
Organizers have dubbed Friday the "Day of Departure," in hopes that, finally, their protests would succeed in compelling Mubarak to leave.
Anxious to move Mubarak out of the way, but wary of appearing to try to impose a plan, the White House authorized a senior official to deny media accounts that the administration had formulated a specific proposal for the contours of a new government. "It's simply wrong to report that there's a single U.S. plan that's being negotiated," the official said.
In an interview Thursday, Mubarak told ABC that he had told President Obama: "You don't understand the Egyptian culture and what would happen if I step down now." The 82-year-old authoritarian, in power for three decades, has said he will leave office after elections that are scheduled for later this year.
After standing in long lines to pass through security checkpoints, thousands upon thousands of Egyptians entered the vast, open square and performed the weekly Friday prayers, kneeling and prostrating themselves in accordance with the muezzin's call.
When the huge throngs finished praying, they began to chant.
"Down, down with Hosni Mubarak!"
"We won't go, he will go!"
"Today is the last day!"
Prime Minister Ahmed Shafiq said late Thursday that Egypt's interior minister should not disrupt Friday's protests. Also Thursday, the government sought to shift the blame for the violence that has swept the capital, and said it was willing to talk with the Muslim Brotherhood, Egypt's oldest and best-organized opposition group.
Omar Suleiman, Egypt's new vice president, suggested that the continuing clashes between Mubarak's supporters and his critics had been instigated by people pursuing a "special agenda" - perhaps from abroad, perhaps from certain business interests, perhaps from the Muslim Brotherhood. "They can be intermingled and interlinked," Suleiman said.
Vice President Biden later telephoned Suleiman, the White House said, to stress that "the Egyptian government is responsible for ensuring that peaceful demonstrations don't lead to violence and intimidation."
The White House also said Egypt would be accountable "for allowing journalists and human rights advocates to conduct their important work, including immediately releasing those who have been detained."
In what the U.S. State Department called a "concerted campaign to intimidate," several dozen journalists were rounded up by security forces and detained for hours Thursday, along with foreigners working as teachers, engineers and human rights researchers. Across the city, angry bands of Mubarak supporters also beat journalists; several reporters said that they were threatened with death.
In his phone call with Suleiman, Biden repeated the administration's call for a transition government, formed of elements of the Mubarak regime - if not Mubarak himself - and representatives of leading opposition groups, with the Egyptian army playing a central role as guarantors.
Among the journalists detained in Cairo were two employees of the New York Times and four from The Washington Post. Leila Fadel, The Post's Cairo bureau chief, and Linda Davidson, a staff photographer, were released Thursday evening; Sufian Taha, a Post interpreter, and Mansour el-Sayed Mohammed, a driver, were released early Friday. The Times employees were also released.
A Dutch reporter was stabbed with a screwdriver. Security officers raided the Ramses Hilton hotel, near Tahrir Square, and confiscated transmission equipment from news organizations that had booked rooms there. Others arrested included three journalists working for al-Jazeera, the satellite television station based in Qatar that has been a target of Mubarak's wrath.
The effort by Egypt to turn the focus on what it regards as its critics came as violence continued to flare in and around Tahrir Square. Some anti-government protesters had converted the offices of a travel agency into a makeshift detention center, where they were keeping captured Mubarak supporters.
Among their captives Thursday morning were two burly men who were stripped to the waist and seated on the floor; their captors said they had been caught with identification cards of the police or the ruling National Democratic Party. One of those detained yipped in pain as the protesters yanked upward on his arms, which were secured by plastic handcuffs.
"Drama queen," one of the captors said. The captors said the prisoners would be turned over to the military once their identities were established.
"Our issue is simple: freedom and social justice," said Hamad Othman Edeep, a 31-year-old teacher who was among those still encamped in the downtown square. He said he makes just under $100 a month, while Egypt's rich grow ever wealthier. "How can I feed my children? Do I have to be a thief?"
As the Egyptian unrest continued, U.S. officials were studying the components of the $1.5 billion annual U.S. aid package to Egypt, and the possibility that it could be used as leverage with the government. But most of the aid - $1.3 billion - is in the form of credits to purchase U.S. military equipment. Any cutoff would not only affect U.S. business but risk offending the Egyptian army, which the administration has praised for its nonpolitical conduct during the crisis.
Suleiman said that Mubarak's son Gamal, who had been considered a likely successor, would not be a candidate to succeed his father. The vice president also said he had begun holding talks with a variety of opposition figures in order to reach consensus on reforms and had invited the banned Muslim Brotherhood to join in. But he insisted that it would take time to get the reforms right.
He thanked the young people who launched the protests here last week, but he said their demand that Mubarak leave office before elections eight months from now was unreasonable.
Shafiq, the newly appointed prime minister, apologized for the violence of the past two days. Both he and Suleiman maintained that the government had nothing to do with the clashes, which began when supporters of the president poured into the center of Cairo on buses and launched an attack on Tahrir Square with a cavalry charge.
The Health Ministry said Thursday that eight people had been killed and more than 800 wounded in the two days of fighting.
"I don't understand what has happened," Shafiq said. "This is not the nature of the Egyptian people."
"We need to know who was behind" the violent attacks at Tahrir Square, Suleiman said. "We will know. And they will be judged accordingly. Those people have actually spoiled and undermined President Mubarak's work."
Human Rights Watch said one of its American staffers, Daniel Williams, a former journalist who worked for The Post and other news organizations, was among several rights workers taken into custody when police and army personnel raided the Hisham Mubarak Law Center.
Egyptian activists saw the targeting of the law center as evidence of a carefully planned attempt to weaken and discredit the pro-democracy movement. The center provides legal representation for a wide range of dissidents and political prisoners, and its lawyers were expected to help defend opposition figures who have been detained during unrest in the last two weeks.
Bahey Eldin Hassan, director of the Cairo Institute for Human Rights Studies, said the presence of foreigners at the offices of Egyptian human rights groups reinforces the view that Westerners are "collaborating against the government of Egypt."
Wilgoren reported from Washington. Staff writers Karen DeYoung and Joby Warrick in Washington contributed to this report.