Assange, currently in custody in the UK, is wanted in the US on a score of charges, including espionage.
The Australian national is due to appear before a London court on Friday for an extradition hearing to the US, which has accused him of conspiring to intercept Pentagon computers, Efe news reported.
In May, a grand jury in Virginia filed 18 new charges against him, including espionage and publication of highly classified documents, which could lead to up to 170 years' sentence, reports say.
The UK Home office said in a statement Assange was arrested in relation to a provisional extradition request from the US. "He is accused of offences, including computer misuse and the unauthorised disclosure of national defence information," it said.
"We have received the full extradition request, which has been certified by the Home Secretary. This case is now before the courts and it would be inappropriate to comment further, it said."
Assange is contesting his extradition to the US stating that he had a commitment to journalism "that has protected many people".
The 47-year-old was sentenced to a 50-week prison by the Southwark Crown Court for violating conditions of parole in 2012.
Assange grabbed international attention in 2010 after WikiLeaks published leaked military information provided by former US soldier Chelsea Manning, which prompted the American government to launch a criminal investigation.
In November 2010, Sweden requested his extradition over allegations of sexual assault and rape, which he denied. He also expressed concern that he would be extradited from Sweden to the US because of his role in the Manning leaks.
Assange surrendered himself to the UK police around a month later and was held in custody for 10 days before being released on bail. When he was unable to challenge the extradition proceedings, he breached bail and in August 2012 was granted asylum by Ecuador.
He remained sheltered in Ecuador's embassy in London for seven years to avoid arrest.
Sweden has since withdrawn its extradition request, but a UK arrest warrant remained in force as Assange had failed to surrender after breaking bail conditions.