In June, the UK approved the extradition of Assange to the United States, where he is facing a possible sentence of up to 175 years on espionage and charges. Assange has appealed the ruling to the High Court in London.
Meanwhile, court documents on Monday revealed that the CIA and former chief Mike Pompeo have been sued for allegedly illegally spying on attorneys and journalists who visited Assange when he stayed in the Ecuadorian Embassy in London.
"Julian is still in [London's] Belmarsh prison, he is still on the way to be extradited to the United States where the government of the United States can kill him in private," Waters said. "Merrick Garland, do the right thing. Free Julian Assange at lunch time today, please."
Waters pointed out that those keeping Assange in the prison are making the WikiLeaks founder sicker and pushing him closer to death.
He also called on all the people who gathered and those listening to him to "never, never shut up" and keep doing what they are doing in efforts to free Assange.
"Free Assange," Waters shouted and the crowd shouted back.
The participants were holding banners with slogans like "Free Assange, Free Press," and "Roger, we live your politics." Other signs called on the Justice Department to stop oppressing the press and drop the charges.
Waters was greeted with applause and lots of by-standers stopped to listen to his address. Wearing casual clothes he spoke without any notes.
During the speech Waters was standing on a banner with the sign of the Justice Department which said the department was "criminalizing journalism."
During the rally, a small truck car was driving back and forth with sayings on its sides like "First Amendment. If he goes it Goes. Save him. Save it." "US commits war crimes in Iraq and Afghanistan and then persecutes the journalist who exposed it. Jail the killers not the messenger."
Waters made the appearance while being on his 2022 "This Is Not a Drill" Tour. Some US politicians have been critical of Waters for his recent comments accusing President Joe Biden of fueling the conflict in Ukraine.