Scotland Yard has confirmed that Julian Assange has been arrested by police in England.
A warrant had been processed by the UK's Serious Organised Crime Agency and was sent by the Metropolitan Police.
Mr Assange, believed to be residing in South-east England, is a suspect in a case in which two women claim they were sexually attacked by him when he visited Sweden in August this year.
The 39-year-old Australian was held when he attended a central London police station by appointment.
Earlier his lawyer had said that Mr Assange would "certainly" oppose deportation on the basis that it could lead to him in turn being handed over to the US.
It is expected he will appear before an extradition hearing at Westminster Magistrates' Court today for a ruling to be made on whether or not he should be sent to Sweden.
Yesterday, US prosecutors were weighing their own legal options in the wake of the latest WikiLeaks disclosures, which America said had put Western security at risk.
The US Attorney General Eric Holder said the Obama administration was considering using laws, in addition to the 1917 Espionage Act, to prosecute him over the release of sensitive information by WikiLeaks.
"That is certainly something that might play a role, but there are other statutes, other tools at our disposal," Mr Holder told reporters.
"The lives of people who work for the American people have been put at risk. The American people themselves have been put at risk by these actions that I believe are arrogant, misguided and ultimately not helpful in any way. We are doing everything that we can." He also disclosed that he had authorised "significant" actions against WikiLeaks.
WikiLeaks has released hundreds of internal US diplomatic messages, some of which contained classified information that has embarrassed the Obama administration and foreign governments.
Although Mr Assange is an Australian citizen, legal experts said last night that it would still be possible to put him on trial in America. In some states it is easier to prosecute people for unauthorised disclosure of classified information. Mr Assange and WikiLeaks, however, could counter that they are akin to a news organisation, protected by the First Amendment of the US Constitution.
But more pressing for Mr Assange is the case being prepared by Swedish prosecutors, who issued a warrant for Assange's arrest last month, but could not enforce it because of a technical blunder in the completion of the document. His details were also added to Interpol's "most wanted" website, alerting police worldwide.