Authorities in Egypt were preventing opposition figure Mohamed ElBaradei from leaving a mosque in Giza, where he had been attending Friday prayers, broadcaster Al Jazeera reported.
The news came as tens of thousands of protesters took to the streets of Egypt's main cities to demonstrate against the government, DPA reported.
Several media outlets had earlier reported that ElBaradei had been detained.
There were also reports that protesters were gathering outside Hosny Mubarak's presidential palace in Cairo.