Protesters, many moved by a Google executive's tearful account of his detention by security forces, poured into Cairo's Tahrir Square to pack a space that can hold a quarter of a million people.
While the government refuses to budge on the demonstrators' main demands, Vice President Omar Suleiman promised there would be no reprisals against the protesters for their three-week-old campaign to eject Mubarak, 82, after 30 years in office.
But they dismissed his promises, accusing the government of playing for time, and swore they would not give up until the current "half revolution" was complete.
By bringing at least as many people onto the streets as the last big demonstration last Friday, the protesters showed their movement has not lost momentum. The next big test will be whether as many people can be mobilized on Friday.
U.S. Vice President Joe Biden also added pressure, pressing Suleiman for an orderly transition of power that is "prompt, meaningful, peaceful and legitimate," the White House said, and calling for the immediate lifting of Egypt's emergency law.
Freed Google executive Wael Ghonim addressed the cheering crowd.
"You are the heroes. I am not a hero, you are the heroes," said Ghonim, who broke down as he described being blindfolded during 12 days of detention.
Ghonim has for now at least been thrust to the forefront of a protest movement that has yet to produce a leader. Activists say Ghonim was behind a Facebook group that helped to inspire the wave of protests. His interview also appears to have persuaded many Egyptians to side with the demonstrations.
"Ghonim's tears have moved millions and turned around the views of those who supported (Mubarak) staying," website Masrawy.com wrote two hours after the interview. In that short span, 70,000 people signed up to Facebook pages supporting him.
Later Ghonim expressed his sorrow for the victims of the violence that has claimed an estimated 300 lives.
"I saw young people dying and now the president has a responsibility to see what the people demand," he told Reuters.
Google had launched a service to help Egyptians circumvent government restrictions on using the social network Twitter, enabling them to dial a telephone number and leave a voice mail that would then be sent on the online service.
The state news agency said 34 political prisoners had been released, the first to be set free since Mubarak promised reforms to quell the popular uprising.
The White House called on Egypt to release all arrested protesters and journalists immediately.
