CAIRO — Protesters demanding President Hosni Mubarak’s ouster packed Cairo’s central square by the tens of thousands today, waving Egyptian flags, singing the national anthem and cheering, appearing undaunted and determined after their camp withstood two days of street battles with regime supporters trying to dislodge them.

The Obama administration said it was in talks with top Egyptian officials about the possibility of Mubarak immediately resigning, and an interim government forming.

Among the options was a proposal for Mubarak to resign immediately and cede power to a transitional government run by Vice President Omar Suleiman.

Thousands of protesters including families with children flowed into Tahrir Square, a sign they were not intimidated after the protesters fended 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.

The crowd was the biggest since Tuesday, when a quarter-million turned out. Thousands prostrated in noon prayers and 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!”

Egyptian Defense Minister Hussein Tantawi visited the square this morning and talked to protesters, the most prominent government official to do so in more than 10 days of unprecedented demonstrations demanding an end of Mubarak’s nearly 30-year rule.

Protesters labeled the rally the “day of leaving,” a reference to their demand Mubarak go today. Some held up signs reading, “Now!”

Mubarak insists he will serve out the remaining seven months of his term. He told ABC News he wants to step down but that doing so would spark chaos, and he vowed not to leave Egypt.