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).