Tens of thousands have packed central Cairo on what they dubbed "the day of departure" in their campaign to remove President Hosni Mubarak.
Protesters waved flags and sung the national anthem, emboldened in their campaign after they repelled pro-regime attackers in two days of bloody street fights.
The US was pressing Egypt for a swift start toward greater democracy, including a proposal for Mr Mubarak to step down immediately.
Thousands including families with children flowed over bridges across the Nile into Tahrir Square.
Protesters in the square held up signs reading "Now!", massing around 100,000 in the largest gathering since the quarter-million who rallied on Tuesday.
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 Mr Mubarak: "Leave! Leave! Leave!"
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 clamour. But protesters throwing rocks pushed them back.
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," Mr Mubarak said he told US President Barack Obama. He warned in an interview with ABC News that chaos would ensue.