Syria was plunged into communication darkness on Thursday when Internet connectivity stopped at midday. Land lines and mobile phone networks were also seriously disrupted.
The Syrian government said "terrorists" had attacked internet lines, but the opposition and human rights groups suspect it to be the work of the authorities.
Opposition activists have used the internet extensively to further their cause by publishing footage of aerial strikes and graphic images of civilian casualties.
In the absence of a free press, they have used social media to disseminate information during the uprising and communicate with journalists abroad.
Anonymous, a loose affiliation of hacking groups that opposes internet censorship, said it will remove all web assets belonging to Assad's government that are outside Syria.
Meanwhile, rebels have seized a series of army bases across Syria this month, exposing Assad's loss of control in northern and eastern regions – and on Thursday fighting on the outskirts of the capital blocked access to the airport.
More than 40,000 people have been killed since the uprising began in March 2011.