Protesters sang Happy Birthday at the sit-in
They were protesting at plans to close the city's oldest purpose-built picture house, exactly 80 years after it opened.
The Save the ABC Cinema group staged the anniversary protest after the last screening of the Harry Potter film.
The cinema is due to close on 16 December.
The building has been put up for sale with a clause saying it can no longer be used as a cinema.
Organisers of the protest asked supporters to book tickets to the Harry Potter film, and to remain at the venue "for a time" afterwards.
The cinema is currently on the market and there are plans to turn it into a health club.
The battle to save the former ABC, which is Bristol's oldest, working, purpose-built cinema, has attracted a lot of support from people in the city.
Hattie Appleby, from the Save the ABC campaign group, believes the ABC is a viable business.
Historic features
"I think potentially it has a good future," she said.
"The stumbling block is that although it is on the market it is not moving anywhere."
In January, Bristol City Council is due to look at applications for alterations to the listed building, which are necessary if it is to be converted into a leisure facility.
"There is a very good chance they won't get this consent and meanwhile there is an offer on the table from a local man, wanting to keep the cinema as a cinema," said Ms Appleby.
Fading popularity
The Odeon group, which owns the ABC, is in the running to manage a new multiplex planned for the Harbourside area of the city.
From the outside the ABC cinema looks much as it did when it opened in 1921, despite the addition of two smaller screens in the 1970s.
It was once ABC's flagship location in the city, but with the advent of the multiplexes its popularity declined.
There are now six cinemas around Bristol, and owners the Odeon group felt the Whiteladies Road venue had had its day.