Ethiopia has blocked social media sites for the next few days, after questions from end-of-year exams were posted online last month, sparking a national scandal and leading to their annulment.
A government spokesman said the ban was aimed at stopping students taking university entrance exams this week from being "distracted."
Facebook, Twitter, Instagram and Viber have been inaccessible in the Horn of Africa nation since Saturday morning.
"It's blocked. It's a temporary measure until Wednesday. Social media have proven to be a distraction for students," government spokesman Getachew Reda told AFP.
Ethiopia is one of the first African countries to censor the internet, beginning in 2006 with opposition blogs, according to experts.
Prominent blogger Daniel Berhane denounced the move as a "dangerous precedent.
"There's no transparency on who decides why it's necessary or who decides for how long," he said.
"This time it's for a few days but next time it could be for months (...) They're flexing their muscles. They got multiple tools and they're testing them."
Last week, the UN Human Rights Council passed a resolution condemning the blocking of internet as a human rights violation.