"Our demand both to the government of Ecuador and the government of the UK is: don't extradite Assange to the US!" the petition, published on the official website of DiEM25, said.
The petition's authors stress that Assange's leaks of classified US government data were in the public good and his arrest posed a serious threat to the freedom of media worldwide.
"The sentence is already clear: if not the death penalty then life in a supermax prison and ill treatment like Chelsea Manning. The very extradition of Julian Assange to the United States would at the same time mean the final death of freedom of the press in the West", the petition added.
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The petition was launched even before Assange was arrested on Thursday at the Ecuadorian Embassy in London, where he has been taking refuge for the past seven years, fearing extradition.
"Freedom of speech-the right to express opinions without government restraint-is a democratic ideal that dates back to ancient Greece. In the United States the First Amendment guarantees free speech though the United States like all modern democracies places limits on this freedom", Anderson tweeted on Friday.
Manning was later dishonourably discharged from the military and served several years in prison for the data leak. In March, she was once again placed in detention after refusing to testify about her release of classified material to WikiLeaks, saying she had already revealed everything during her trial.