Opposition media outlets said the block was implemented by Erdogan's government, after the prime minister told a rally he would eradicate Twitter access in the country.
There was no official confirmation from the government.
But the website for the country's telecommunications authority (TIB) turned up four separate court rulings referencing "twitter.com" that suggested official action had been taken.
One of them says: "The protection measure has been taken for this website (twitter.com) according to the decision... of the Istanbul chief public prosecutor's office and has been implemented by the TIB."
In Thursday's rally to drum up voter support ahead of March 30 local elections, Erdogan had told supporters: "We will wipe out Twitter. I don't care what the international community says."
Erdogan's office said in a statement that Twitter remained "indifferent" to Turkish court rulings demanding "some links" be removed and that the premier was turning his attention to the matter.
The state-run Anatolia news agency said the only solution to "address the unjust treatment of our citizens" deemed in violation of Turkish law was to employ technical measures to block access to Twitter.
Erdogan, Turkey's charismatic and increasingly autocratic leader since 2003, has come under mounting pressure since audio recordings spread across social media that appeared to put him at the heart of a major corruption scandal.