Assange, who has been holed up in the Ecuadorean Embassy since June to avoid extradition, made the comments at a packed event on the sidelines of the United Nations general assembly.
Assange mocked Obama for defending free speech in the Arab world in an address to the United Nations on Tuesday, pointing to his own experience as evidence that Obama has "done more to criminalise free speech than any other US President."
"It must have come as a surprise to the Egyptian teenagers who washed American teargas out of their eyes (during the Arab Spring) to hear that the US supported change in the Middle East," Assange said. "It's time for President Obama to keep his word ... and for the US to cease its persecution of WikiLeaks," he said.
Assange's combative comments, plus statements made by Ecuadorean foreign minister Ricardo Patino and his other allies at the event, suggested no solution is in sight to the diplomatic standoff surrounding the 41-year-old Australian.
Assange addresses a meeting via videolink from Ecuador's London embassy during the United Nations general assembly at UN headquarters. AP/Jason DeCrow
British authorities have surrounded the Ecuadorean Embassy and said if Assange sets foot outside, they will arrest him and extradite him to Sweden to face rape and sexual assault allegations.
Assange's lawyers and Ecuador's government fear that could lead in turn to extradition to the United States, where they say he would face "inhumane" prison conditions and even the death penalty.
Assange, who looked to be in good health as he sat at a desk in front of a bookshelf and addressed the 150 or so people at the event, said Britain and Sweden have so far refused to provide guarantees he would not be extradited to the United States.
US and European government sources have countered that the United States has issued no criminal charges or launched any attempts to extradite Assange.
US President Barack Obama point to people on the South Lawn of the White House in Washington. AP Photo/Carolyn Kaster
In Britain's court
Patino is scheduled to meet with British Foreign Secretary William Hague in New York on Thursday to discuss Assange, and he said there are "multiple paths" that could lead out of the standoff. Yet, in an interview with Reuters following the UN event, Patino made clear that Ecuador is not willing to cede much ground. "The ball's in their court right now," Patino said.
Patino held in his hands a mimeographed copy of an 1880 agreement signed between Britain and Ecuador, which he said prohibits extradition in cases such as Assange's. He said he would show the document to Hague on Thursday.
Patino rigorously defended Ecuador's decision to grant political asylum to Assange, expressing disbelief that Britain is "determined" to arrest the former computer hacker even though he said there are no criminal charges against him. "This means you have reason to suspect he's being persecuted," Patino said.
He said Assange is in relatively good spirits but expressed concern his physical and psychological condition could deteriorate.
"I think of myself, how I'd react in that situation, not being able to go outside, being isolated," Patino said. "It's practically like being jailed."
Full text: Obama's speech at UN general assembly
Obama exploiting Arab Spring in campaign, says Assange
Julian Assange accused US President Barack Obama of seeking to exploit the Arab uprisings for personal political gain, as he addressed a sideline meeting of the UN general assembly via videolink from his hideout at Ecuador's embassy in London. At a sideline meeting organised by Ecuador, the activist attempted to draw parallels between himself and the instigators of the Arab Spring claiming that they had all been let down by Obama. Read more
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