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".
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 08:00 (05:00 GMT). People in Istanbul were unable to access any 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, giving no further details.
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.
(Published by BBC - April 29, 2017)