Assange was arrested at a London police station at 9:30 a.m. (4:30 a.m. ET) and will appear at the City of Westminster Magistrate's Court later in the day, police said.
Swedish authorities had issued the warrant for Assange so they can talk to him about sex-crime allegations unrelated to WikiLeaks' recent disclosure of secret U.S. documents.
Assange, a 39-year-old Australian, has said he has long feared retribution for his website's disclosures and has called the rape allegations against him a smear campaign.
Sweden first issued the arrest warrant for Assange in November, saying he is suspected of rape, sexual molestation and illegal use of force.
Last week, at the request of Sweden's Stockholm Criminal Court, Interpol issued a "red notice" placing Assange on a list of wanted suspects.
British police then asked Swedish authorities for additional details not specified in the initial arrest warrant, a possible indication that the location of the elusive Assange is known. CNN has not confirmed that Assange is in the United Kingdom.
Swedish prosecutors said Monday that they had sent additional information the British requested and that the case was being handled in accordance with European laws.
WikiLeaks, which facilitates the anonymous leaking of secret information, has been under intense pressure from the United States and its allies since it began posting the first of more than 250,000 U.S. State Department documents on November 28.
Since then, the site has been hit with denial-of-service attacks, been kicked off servers in the United States and France, and found itself cut off from funds in the United States and Switzerland.
In response, the site has rallied supporters to mirror its content "in order to make it impossible to ever fully remove WikiLeaks from the internet," with more than 500 sites responding to the appeal by Monday evening, it said.
CNN
