Julian Assange called Sunday for US soldier Bradley Manning, the alleged source of a massive trove of secret government documents published by WikiLeaks, to be released from a military prison.

Assange made the appeal in a speech from the balcony of the Ecuadoran embassy in London, his first address since he was granted political asylum in his long-running bid to avoid extradition from Britain to Sweden.

He said Manning "was found by the United Nations to have endured months of tortuous detention" at the US Marine Corps jail in Quantico, Virginia.

"He has yet, after two years in prison, to see a trial. He must be released," Assange said to applause from around 100 of his supporters who gathered outside the embassy to hear him speak from a balcony.

"If Bradley Manning did as he is accused, he is a hero and an example to all of us," Assange said, hailing him as "one of the world's foremost political prisoners".

Manning, 24, faces a possible life sentence for "aiding the enemy" by releasing tens of thousands of classified files about the wars in Iraq and Afghanistan to WikiLeaks, which portrays itself as a whistleblower website.

The US army private was arrested in Iraq in 2010.

His trial is tentatively due to begin later this year but could be pushed back to as late as February.

In his speech, Assange also called for US President Barack Obama to stop a "witchhunt" against WikiLeaks.

Assange is wanted in Sweden to answer questions about alleged sex crimes, but he insists he will eventually be extradited to the United States and face trial for divulging state secrets.

The US State Department said Thursday it had no intention of "persecuting" Assange and denied charges that it was pressuring Britain to seize him.