Egypt is trying to clamp down on Facebook and Twitter as youths use the internet to organise more anti-government protests.
Twitter confirmed that its service has been blocked in Egypt on Tuesday from around 1600GMT.
A Swedish mobile video site called Bambuser also reported that it had been blocked around the same time.
However, the Facebook page used to co-ordinate many of the protests has remained online.
The 6th of April Youth Movement used its Facebook page to urge protesters to continue Wednesday against President Hosni Mubarak’s decades-old rule.
Facebook did not reveal whether it had implemented any technical measures to keep the site running but a spokesperson told the BBC that it was “seeing consistent levels of traffic”.
The social network, which has more than 600m users, recently was forced to intervene when it emerged that political protest pages in Tunisia were being hacked and passwords stolen, seemingly at the behest of the former government.
Egypt’s Interior Ministry said on Wednesday that no new demonstrations would be tolerated. It warned that protesters would be prosecuted, if caught.
Throngs of rock-throwing demonstrators occupied Cairo’s Tahrir Square for hours on Tuesday, smashing back attempts to dislodge them by police tear gas and water cannons. Several thousand people demonstrated in Alexandria, and there were reports of large protests in other cities including Mansoura and Mahalla al-Kobra.