Turkey has blocked all access inside the country to the online encyclopaedia Wikipedia.
Officials said "an administrative measure" had been taken, but gave no reason why.
Turkish media said authorities had asked Wikipedia to remove content by writers "supporting terror" but the site had not responded to the demands.
Turkey has temporarily blocked social media sites including Facebook and Twitter in the past, usually following protests or terror attacks.
The Turkey Blocks monitoring group said Wikipedia was unreachable from 8am (local time) and people were unable to access any pages without using a Virtual Private Network (VPN) which obscures where a person is accessing the internet from.
"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 said.
However, the Hurriyet daily newspaper said Wikipedia had been asked to remove content by certain writers whom the authorities accuse of "supporting terror" and of linking Turkey to terror groups. The site had not responded to the demands, Hurriyet said, and the ban was imposed as a result.
Turkey Blocks and Turkish media, including Hurriyet, said the provisional order would need to be backed by a full court ruling in the next few days.
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".
The Turkish government has previously denied censoring the internet, blaming outages on spikes in usage after major events.
Wikipedia has also faced censorship in other countries, including a temporary ban in Russia, and repeated crackdowns in China.