The Ethiopian government has blocked social media sites across the country to control distraction among students who were scheduled to take university entrance exams.
The blocked sites include Facebook, Twitter, Instagram and Viber and they will be unavailable for several days while university entrance exams are taken, a government spokesman said.
"It's a temporary measure until Wednesday. Social media have proven to be a distraction for students," said spokesman Getachew Reda.
Ethiopia was among the first African countries to censor the internet and opposition blogs and human rights websites are frequently blocked.
Social media sites have gone down in Ethiopia before but only for a matter of hours, with the government previously denying any involvement. This is the first time social media sites have been publicly blocked nationwide.
Daniel Berhane, the editor of Horn Affairs magazine, said on Twitter: "This is a dangerous precedent. There is no transparency about who took the decision and for how long."
In May, university entrance exams were cancelled after pictures of the test circulated on social media. A group supporting protests for greater rights for Ethiopia's Oromo people claimed responsibility for the leak.
Traditional media in Ethiopia are tightly controlled by the government, leaving many reliant on social media to access and pass on information critical of the authorities.