"The U.S. war on whistle-blowers must end," said Assange, who emerged from the Ecuadorian Embassy in London for his first public appearance in months.
"As WikiLeaks stands under threat, so does the freedom of expression and the health of all our societies," Assange told reporters and a group of cheering supporters.
Assange called on the United States to release U.S. soldier Bradley Manning, who is accused of giving WikiLeaks hundreds of thousands of pages of military and diplomatic documents. He also spoke in support of the politically charged Russian rock band Pussy Riot and Bahrani activist Nabeel Rajab, CNN said.
Assange was recently granted asylum by Ecuador to avoid being deported from Britain to Sweden to face sexual assault charges. His lawyer this week demanded Britain grant Assange safe passage to South America.
Assange's status has become an international political football with the Ecuadorian government warning Britain not to try to arrest Assange.