Turkey has blocked all access inside the country to the online encyclopaedia Wikipedia, one of the world’s most popular websites.
It was not initially clear why the ban had been imposed.
The Turkey Blocks group said the site was inaccessible from 08:00 (05:00 GMT) by order of the Turkish authorities.
People in the capital Istanbul were unable to access any Wikipedia pages without using a Virtual Private Network (VPN).
“After technical analysis and legal consideration based on the Law Nr. 5651 [governing the internet], an administrative measure has been taken for this website,” Turkey’s Information and Communication Technologies Authority was quoted as saying.
No reason was given.
Turkey Blocks and Turkish media, including the Hurriyet Daily News, said the provisional order would need to be backed by a full court ruling in the next few days.
Another day, another outage – by Mark Lowen, BBC Turkey Correspondent
It’s become all too familiar here: the endless ‘loading’ icon followed by the message ‘server timed out’.
Blocking websites is a common tool of the Turkish authorities: Twitter, Facebook and YouTube have suffered the same fate several times, and numerous anti-government sites are inaccessible.
Critics say it smacks of Turkey’s repression of free speech: over half of all requests to Twitter to remove content have come from Turkey, and the country now ranks 155 of 180 in the press freedom index of the watchdog Reporters without Borders.
Social media was in uproar as news of the ban emerged, with some users speculating that it might be a bid to suppress criticism on President Recep Tayyip Erdogan’s Wikipedia page.
Mr Erdogan narrowly won a controversial 16 April referendum on increasing his powers, but the issue has deeply divided the country.
One Twitter user noted that the Wikipedia page on Turkey’s referendum has a section on “controversies and electoral misconduct”, and cites claims that the government suppressed the ‘No’ campaign through “arrests, control of the media and political suppression”.
Turkey has temporarily blocked popular social media sites including Facebook and Twitter in the past, especially in the wake of mass protests or terror attacks.
The government has previously denied censoring the internet, blaming outages on spikes in usage after major events.