The crowds - determined but peaceful - filled Tahrir, or Liberation, Square and spilled into nearby streets, among them people defying a government transportation shutdown to make their way from rural provinces.
Protesters jammed in shoulder-to-shoulder, with schoolteachers, farmers, unemployed university graduates, women in conservative headscarves and women in high heels, men in suits and working-class men in scuffed shoes.
They sang nationalist songs, danced, beat drums and chanted the anti-Mubarak slogan ''Leave! Leave! Leave!'' as military helicopters buzzed overhead.
Organisers said the aim was to intensify marches to get the president out of power by Friday, and similar demonstrations erupted in at least five other cities around Egypt.
Image: Young girls wave Egyptian flags atop an armoured vehicle just outside Tahrir or Liberation Square in Cairo, Egypt, on Tuesday, Feb 1, 2011. (Photographs copyright PTI)