The rally featured Mayor Ed Lee, Democratic Leader Nancy Pelosi, as well as Supervisors Malia Cohen and Norman Yee and Sen. Scott Wiener, among others.
"Now that we are here to defend the Affordable Care Act we remember the words of Martin Luther King. He said of all the forms of inequality, injustice in health is the most shocking and inhumane," said House Democratic Leader Nancy Pelosi.
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The rally was part of a national day of action in which simultaneous rallies were held in more than 40 cities across the country, including Los Angeles and Sacramento.
"Ain't going to let no ego maniac turn me around, turn me around, keep on walking ,keep on talking going to build ourselves a brand-new world," singer Joan Baez said.
There were many senior citizens in the crowd worried about the new president cutting programs they depend on.
"I'm worried that Medicare benefits will be decreased and more of the burden will be carried by each individual person," Trump transition nurse Mary Roth said.
But for all these Trump detractors, there are those who can't wait for the demise of the Affordable Care Act.
"It's already gone. It's a done deal. Now they're going to take six months to design a new program that will work for the entire country," Trump transition Hispanic Republican Assemble Chair Leo Lacayo said.
More health care advocates gathered in Richmond where other congressional leaders launched what they're calling #OurFirstStand here and at the big rally in San Francisco
"A repeal vote will start us down the path on the road to chaos in the insurance market and declines in coverage," San Francisco Mayor Ed Lee said.
The National Nurses United also took part in the San Francisco rally, citing millions of Americans who would be cut off from access to healthcare if the ACA, Medicare and Medicaid were dismantled.
"On this day of action we are standing with our elders, our friends and family, along with many of our elected representatives to say no to the Republicans' disastrous proposals," National Nurses United Co-president Deborah Burger said in a statement.
A huge march in defense of the Affordable Care Act is being planned for February 1 in San Francisco.
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Bay City News contributed to this story.