Egypt appears to have clamped down on web services, such as Twitter, that have been used to help organise anti-government protests in Cairo.
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.
Facebook has not said whether it has implemented any technical measures to keep the site up and running but a spokesperson said 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.
The site implemented a series of technical measures to counter the attacks, including encrypting all requests for the site from within Tunisia.
'Right to protest'Details of the blocks in Egypt began to emerge on Tuesday afternoon, as thousands of people joined a "day of revolt" against the government of President Hosni Mubarak.
Initially it was unclear whether state authorities were blocking internet access or if mobile networks were simply overloaded by the numbers of people gathering in the streets.
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The BBC's Jon Leyne, who is in Cairo, said the protests like this in Egypt were rare
<!-- END - caption -->Twitter were at first unwilling to comment, instead directing people towards Herdict, a website created by a group at Harvard University, which collects reports of websites that are down.
The site has seen a small spike in the number of reports about Twitter from Egypt.
But overnight, Twitter issued a statement..
"We can confirm that Twitter was blocked in Egypt around 8am PT today. It is impacting both Twitter.com and applications."
The block had previously been confirmed by BBC readers and telecom operator Vodafone Egypt, which sought to reassure customers that it was not responsible.
"We didn't block Twitter - it's a problem all over Egypt and we are waiting for a solution," it said.
Some Twitter messages - many tagged #jan25 to show they were in support of the demonstrations - seemed to have got through via text messages and some third-party applications.
Bambuser, a service that allows people to stream live video from a phone to a website or a Facebook page, also said that it had been hit by the ban.
"We are working hard to ensure access resumes for the Egyptian community and stand by them in their quest for the right to protest," it said.
Street banIn support of the protests, a collective of online "hacktivists" known as Anonymous claimed to have taken down the Egyptian interior ministry's web page.
Anonymous came to prominence for its cyber-attacks on the websites of companies it deemed to be anti-Wikileaks.
It has since turned its attention to supporting the protests in Tunisia and Egypt.
The group uses so-called "denial-of-service" attacks, that seek to swamp websites with large volumes of traffic, until they are knocked offline.
The group was one of many that also offered advice to the protesters on how to evade blocks on sites and services.
However, some sites appear to have escaped the block.
Google-owned video sharing website YouTube, which has been used to share film of the protests, has seemingly not been blocked.
Protesters are also turning to other digital tools to help.
For example, a map has been set up by the Arabic Network for Human Rights Information, for people to report arrests, and harassment during the protests.
The digital blocks come as the government issued a ban on street protests.
Public gatherings, protests and marches are all now prohibited, the country's official news agency reports.
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