Defence Minister Hussein Tantawi and senior army officials visited the square in a sign that Egypt's most powerful institution was sanctioning the demonstration. Soldiers checked IDs and performed body searches at entrances to the Square
The Obama administration said it was in talks with top Egyptian officials about the possibility of Mubarak immediately resigning, and an interim government forming before free and fair elections this year.
The creation of a military-backed caretaker government was one of several ideas being discussed, said the officials, who spoke on condition of anonymity to discuss the sensitive diplomatic talks that are continuing.
Among those options is a proposal for Mubarak to resign immediately and cede power to a transitional government run by Vice President Omar Suleiman.
Human chains of protesters performed secondary searches inside Tahrir Square, where the atmosphere was relaxed.
Pro-Mubarak crowds that have attacked demonstrators and foreign journalists did not have a visible presence.