YouTube was among the websites to be banned
Libya must stop blocking access to opposition websites and internet pages such as YouTube, US-based Human Rights Watch has warned.
The activists say Tripoli began a crackdown on 24 January, blocking several foreign-based sites reporting on Libya, and the entire YouTube site.
"The government is returning to the dark days of total media control," the group said in a statement.
In December, the group praised Libya for allowing reporters to work there.
But they said the latest move was a "disturbing step away from press freedom".
"These websites were the one recent sign of tangible progress in freedom of expression in Libya," said HRW's Sarah Leah Whitson.
"Libya can stick its head in the sand and try to block the free flow of electronic information to its citizens, but the good news is we all know they'll fail."
"Whether in China or Saudi Arabia or Libya, citizens will always find ways to exchange knowledge and information, with or without their government's consent."
The BBC's Rana Jawad in Libya says internet users were initially told that the problems were a technical fault.
She adds that a step-by-step guide on how to access the blocked YouTube site from Libya has emerged on the pages of some local users of the social networking site Facebook in recent days.
She also says that a Facebook group page called "We want YouTube back in Libya" has been set up, with more than 2,000 "fans".
HRW said blocking websites was contrary to international agreements Libya had signed up to.