Hundreds of demonstrators who camped outside the State Security Investigations agency since Friday ran unchecked throughout the building, grabbing computers and document-filled file cabinets, the al-Masry al-Youm news outlet reported.
The report said the agency was widely feared and there were widespread allegations of political torture. No one from the agency made any remarks to the media.
Security forces offered little resistance as the crowds surged into the building Saturday, local media said.
Several demonstrators told The Washington Post they found "implements used for torture, including electric shock devices" in the building.
The reformist uprising that saw longtime autocratic President Hosni Mubarak abandon office last month created diplomatic concerns in the United States and other Western countries that considered Egypt an ally, particularly in the area of anti-terrorism.