Left Unity, a broad left political party formed in 2013, said it had chosen the venue to highlight the number of large buildings sitting empty in London.
The launch happened on March 31 at a building in Soho.
Felicity Dowling, who is standing as a Left Unity General Election candidate in Ellesmere Port and Neston, said: “This is a manifesto launch with a difference. Instead of using a glitzy venue like the main parties, we want to highlight the growing crisis of homelessness.
“Rents are rocketing and good housing getting further out of reach for so many, yet there are 700,000 empty buildings in Britain that could be brought back into use.
“It’s just not right that so much space is wasted while people are sleeping on the streets.”
Paul, from the Love Activists London group, which is occupying a number of buildings in London, added: “Our aim is to occupy prominent buildings in the central London area to highlight the issues of homelessness, gentrification and capitalism.”
Nationally, Left Unity is planning to stand candidates in around 10 constituencies, mainly where Labour has a big majority.
Film director Ken Loach, who co-founded the party, also attended the launch.