CAIRO >> Tens of thousands packed central Cairo Friday, waving flags and singing the national anthem, emboldened in their campaign to oust President Hosni Mubarak after they repelled pro-regime attackers in two days of bloody street fights. The U.S. was pressing Egypt for an immediate start to democratic transition, including a proposal for Mubarak to step down immediately.

Thousands including families with children flowed over bridges across the Nile into Tahrir Square, a sign that they were not intimidated after fending off everything thrown at them by pro-Mubarak attackers — storms of hurled concrete, metal rebar and firebombs, fighters on horses and camels and automatic gunfire barrages.

In the wake of the violence, more detailed scenarios were beginning to emerge for a transition to democratic rule after Mubarak’s nearly 30-year authoritarian reign. The Obama administration said it was discussing several possibilities with Cairo — including one for Mubarak to leave office now and hand over power to a military-backed transition.

Protesters in the square held up signs reading “Now!”, massing around 100,000 in the largest gathering since the quarter-million who rallied Tuesday. They labeled Friday’s rally the “day of leaving,” the day they hope Mubarak will go.

Thousands prostrated themselves in the noon prayers, then immediately after uttering the prayer’s concluding “God’s peace and blessings be upon you,” they began chanting their message to Mubarak: “Leave! Leave! Leave!” A man sitting in a wheelchair was lifted — wheelchair and all — over the heads of the crowd and he pumped his arms in the air.

Those joining in passed through a series of beefed-up checkpoints by the military and the protesters themselves guarding the square. In the afternoon, a group of Mubarak supporters gathered in a square several blocks away and tried to move on Tahrir, banging with sticks on metal fences to raise an intimidating clamor. But protesters throwing rocks pushed them back.

The Arabic news network Al-Jazeera said a “gang of thugs” stormed its offices in continuation of attacks on journalists by regime supporters that erupted Thursday. It said the attackers burned the office and damaged equipment. The editor of the Muslim Brotherhood’s website, Abdel-Galil el-Sharnoubi, told the AP that policemen stormed its office Friday morning and arrested 10 to 15 of its journalists. Also clashes with sticks and fists between pro- and anti-government demonstrators erupted in two towns in southern Egypt.

Various proposals for a post-Mubarak transition floated by the Americans, the regime and the protesters share some common ground, but with one elephant-sized difference: The protesters say nothing can be done before Mubarak leaves.

The 82-year-old president insists he will serve out the remaining seven months of his term to ensure a stable process. “You don’t understand the Egyptian culture and what would happen if I step down now,” Mubarak said he told President Barack Obama. He warned in an interview with ABC News that chaos would ensue.

But the Obama administration was in talks with top Egyptian officials about the possibility of Mubarak immediately resigning and handing over a military-backed transitional government headed by Vice President Omar Suleiman.

Such a government would prepare the country for free and fair elections later this year, according to U.S. officials speaking on condition of anonymity to discuss the continuing sensitive talks. The officials stressed that the United States isn’t seeking to impose a solution on Egypt but said the administration had made a judgment that Mubarak has to go soon if there is to be a peaceful resolution.

Suleiman has offered negotiations with all political forces, including the protest leaders and regime’s top foe the Muslim Brotherhood, over constitutional changes needed to ensure a free vote ahead of September presidential elections to replace Mubarak, who has promised not to run again.

Among them: provisions to ensure independent supervision of elections, a loosening of now suffocating restrictions on who can run for president and a term limit for the president.

Nobel Peace laureate Mohamed ElBaradei, one of the leaders of the protest movement, lay out his scenario on Friday: a transitional government headed by a presidential council of two or three figures, including a military representative.

ElBaradei said he respects Suleiman as someone to negotiate with over the transition, but did not address whether he should have any presidential role.

The protesters in Tahrir have not seemed to have a unanimous view on Suleiman, a military man who was intelligence chief and Mubarak’s top aide until being elevated to vice president last week. Some are willing to see him head any transitional government, others view him as too much of a regime figure and demand he go too.

ElBaradei repeated the protesters’ condition that Mubarak must leave immediately before there can be negotiations with the government over the nation’s future.

“He should hear the clear voice coming from the people and leave in dignity,” ElBaradei told a press conference. “The quicker he leaves in dignity the better it is for everybody.”

But he underlined that the protest movement is not seeking “retribution” or a complete purge. “Not everyone who worked with the regime should be eliminated,” he said. “There will be no severance with the history and past of Egypt.”

There were other potential difference with Suleiman’s scenario. ElBaradei said the constitutional changes must include greater freedom to form political parties, which now effectively need the approval of Mubarak’s ruling party. Protesters also demand the lifting of the emergency law in place for the entirety of Mubarak’s rule, giving security forces near unlimited powers.

Suleiman has mentioned neither issue, though he said the regime is willing to discuss far-reaching changes.

Another issue is time-frame. Suleiman spoke of completing constitutional changes by July to hold presidential elections in September. ElBaradei said that was not enough time to uproot a system that has ruled for decades through a monopoly on politics and widespread election fraud to ensure a proper vote.

“People are not stupid not to understand that this is not really a genuine desire to go for reform,” he said of the July/September schedule.

Instead, he said, the presidential council should rule for a year under a temporary constitution, during which time a permanent document would be drawn up and only afterwards elections held.

One self-professed potential candidate — Arab League chief Amr Moussa — appeared in the square Friday, his convoy greeted by chants of “we want you as president, we want you as president.” Moussa, previously a former foreign minister under Mubarak, has an elder statesman appeal for some Egyptians, boosted by the tough rhetoric he takes on Israel.

Asked earlier by France’s Europe 1 radio if he would consider a role in the transitional government or eventually running for president, Moussa replied, “Why say no?”

Another visitor to the square Friday: Egyptian Defense Minister Hussein Tantawi, who mingled with protesters and held friendly but heated discussions, telling them most of their demands have been met and they should go home. He was the highest level government figure to visit the square in more than 10 days of demonstrations.

At Tahrir, soldiers checked IDs to ensure those entering were not police in civilian clothes or ruling party members and performed body searches at the square’s entrances, a sign that Egypt’s most powerful institution was sanctioning the demonstration.

The atmosphere was peaceful after the 48 hours of violence between pro- and anti-Mubarak crowds battling with rains of rock and concrete torn from the street and shields fashioned out of sheet metal from a construction site. At least eight people were killed in the fighting and more than 800 injured. Gangs backing Mubarak attacked journalists and human rights activists across Cairo Thursday, while others were detained by soldiers.

The pro-Mubarak crowds that have attacked demonstrators and foreign journalists did not have a visible presence in Tahrir on Friday. On the other side of Cairo, dozens of regime supporters carrying machetes and sticks set up an impromptu checkpoint on the ring-road highway encircling the city of 18 million, stopping cars to inspect them and ask for IDs. The roadblock appeared to be looking for protesters heading to Tahrir. One of the armed men wore a sign around his neck reading, “We are sorry, Mr. President.”

In Tahrir, protesters formed their own cordon inside the military’s to perform a secondary check of IDs and bags. Many of those arriving brought fresh bread, water, fruit and other supplies, and the atmosphere was relaxed. Long lines formed at tables of people handing out tea and bread. Many waved the Egyptian flag or chatted amicably with the soldiers. Women in full face veils and enveloping robes stood close to women in blue jeans and tight tops.

Around the square were makeshift clinics, set up in the entranceways of stores, including a KFC. At one, a man received an injection in his arm. Above another was the sign of an interlocking crescent and cross.

Around 5,000 of the protesters prostrated themselves in prayer at noon. Though men and women prayed separately as is traditional, the women knelt in a block parallel to the men instead of behind them out of sight or in a separate area entirely as takes place in most Egyptian mosques. After uttering the concluding “God’s peace and blessings be upon you” of the prayer, they began the chant: “Leave! Leave! Leave!”

A number of celebrities of Egyptian cinema and TV joined the march, including Sherihan, a beloved screen beauty from the 1980s and early 1990s who largely disappeared from the public eye because of health issues. “This is really a popular revolution, it’s civilized and honorable,” she told Al-Jazeera TV.

“We’re calling on this to be the largest protest ever,” said Mahmoud Salem, a youth activist and blogger. “We are hoping it will be the last one.” He said that during Thursday’s turmoil, his car was attacked by regime supporters as he and four friends tried to deliver supplies to the square. He said the rioters relentlessly smashed the car windows and ripping off the side mirrors until he and his colleagues fled from the car.

“It was like a zombie movie,” he said.