Turkey has restricted access to Twitter just hours after Prime Minister Recep Tayyip Erdogan threatened to "wipe out" the social-media network.
Many users trying to access the network were confronted with a notice from Turkey's telecommunications authority citing court orders for the site's apparent closure.
Twitter spokesman Nu Wexler said the company was investigating.
Turkish President Abdullah Gul via his Twitter account
has denounced the government's restrictions.
"A complete ban on social media platforms cannot be approved," Gul said.
EU enlargement chief Stefan Fuele tweeted that he was "gravely concerned" by the block by an aspiring EU country.
U.S. State Department spokeswoman Jen Psaki said Turkey's actions were contrary to "Turkey's own expressed desire to uphold the highest standards of democracy," and that an independent and unfettered media was an essential element of democratic open societies.
Erdogan has come under mounting pressure since audio recordings spread across social media have appeared to put him at the heart of a major corruption scandal.
Based on reporting by AP, Reuters, and RFE/RL