The state-run Anatolia news agency said authorities "technically blocked access to Twitter" because the service had ignored various Turkish court orders to remove some links deemed illegal.
Twitter responded by saying on its official @policy feed that Turks could get around the block by tweeting through mobile telephone text services.
In early reaction, the EU commissioner for digital agenda, Neelie Kroes, tweeted that the block in Turkey "is groundless, pointless, cowardly".
She added that the "Turkish people and international community will see this as censorship. It is."
The restriction of access to Twitter came after Erdogan told a rally drumming up support for March 30 local elections that he would eradicate Twitter access in the country.
"We will wipe out Twitter. I don't care what the international community says," he said.
Erdogan's office said in a statement that Twitter had remained "indifferent" to Turkish court rulings demanding "some links" be removed, and that the premier therefore had turned his attention to the matter.
The website for the country's telecommunications authority (TIB) turned up four separate court rulings referencing "twitter.com".
One of them said: "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."
Anatolia ran a report saying a Twitter block was the only solution to "address the unjust treatment of our citizens".
Graft allegations tweeted
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.
Recordings include an apparent discussion between Erdogan and his son about hiding money, as well as others in which he appears to be interfering in business deals, court cases and media coverage.
Some of the most damaging information has come from a Twitter account under the name Haramzadeler ("Sons of Thieves"), which appears to have access to a huge trove of secret documents and police wiretaps linked to the investigation.
Erdogan has dismissed most of the recordings as "vile" fakes concocted by his rivals, and threatened to ban YouTube and Facebook after crucial local elections on March 30.
"This has nothing to do with freedoms. Freedom does not mean the right to intrude on someone's privacy, or to pass the state's secrets to the international arena," Erdogan said on Thursday.
The prime minister is openly suspicious of the Internet, and last year called Twitter a "menace" for helping organize mass anti-government protests.
A vast corruption probe launched in December saw dozens of people rounded up, including close business and political allies of the prime minister.
The Turkish strongman has accused associates of a former staunch ally – US-exiled cleric Fethullah Gulen – of being behind the graft probe that claimed the scalps of four ministers.
Gulen has denied any involvement.
Turkey recently tightened government control of the Internet and the judiciary, generating criticism from rights groups.
The country, which has more than 10 million Twitter users, has seen access to thousands of sites blocked in recent years.
YouTube was banned for two years up to 2010 because of material deemed insulting to the country's revered founder, Mustafa Kemal Ataturk.
The Internet Publishers Association, a body representing online and media companies, said the move to block Twitter was an attempt to "destroy freedom of expression".
"The prime minister having the power to shut down Twitter will be the confirmation of dictatorship," it said in a statement published by local media.