Chanting slogans of "Free Julian Assange!" and "Free journalist whistle blowers!" the protesters said they did not want to remain silent on the issue, accusing the U.S. government of "attacking" the freedom of speech.
Marilyn Langlois, a community activist in San Francisco who used to work for the local government, said she believed Assange had committed no crime and was being held unjustly and tortured in Britain.
"He has simply published the information that was given to him, information that had evidence of U.S. war crimes 10 years ago, and the American public needs to know about those war crimes that should be prosecuted," she told Xinhua.
Langlois said she hoped the U.S. government would not extradite Assange to stand trial in the United States.
"If we as a nation stand opposed to the concept of war crimes in other countries, we must be opposed to war crimes committed by ourselves," she said.
The U.S. government reportedly formally requested Assange's extradition last June to face charges in the United States, as the U.S. Department of Justice accused him of helping former intelligence analyst Chelsea Manning hack into computers of the U.S. Defense Department.
Having been in exile for years to escape the U.S. government's prosecution, Assange was arrested last April after being forcibly removed from the Ecuadorian Embassy in London, where he had been living since 2012.
The protesters in San Francisco demanded a serious investigation of allegations and evidence of war crimes committed by the U.S. government.
Jax Domnitz, who claimed to be an active member of American Civil Liberties Union, Northern California, said the freedom of speech was attacked and eroded in the United States.
She rejected the U.S. accusation against Assange, asserting that all he did was to report the facts.
"He gave us information about the wars, and what's really kind of ironic is that most people don't even know in this country," she said, adding that people in America should wake up and have their voices heard.
The event's organizers said Monday's rally is part of a global protest in solidarity with supporters in other countries who have risen to speak for the WikiLeaks founder.