The Occupy London protest group says it has taken over a fifth site in London.
Up to 20 demonstrators moved into Roman House, on Fore Street, Barbican, in the City of London, in the early hours of Saturday, an Occupy spokesman said.
The "economic justice" campaigners said they planned to "open the building to the public" on Monday.
Berkeley Homes, which owns the empty office block, has asked protesters to leave, saying the building was "not safe for public use".
The firm said the property was being converted into 90 residential flats and the occupation was putting jobs at risk.
An Occupy London spokesman said the group would leave if there was a building contract in place, because to remain and put jobs at risk would be "so against what we are about".
City of London Corporation and police said they were aware of the occupation.
The 1950s building has been lying empty for the past few years. Protesters said they targeted it because it "previously housed companies from the financial service industries".
Berkeley Homes disputed this claim saying an architecture firm was the previous occupant of the eight-storey building.
The latest move from Occupy comes after the corporation won its High Court case on Wednesday to evict protesters from outside St Paul's Cathedral, where the Occupy London Stock Exchange group set up its tents on 15 October.
Since October the protest group has "occupied" Finsbury Square, an empty office building owned by the Swiss bank UBS in Sun Street, east London, and the empty Old Street Magistrates Court.
A statement on Occupy London's website said it had "publicly repossessed Roman House".
It added: "The Occupy London campaigners - part of the global movement for social and economic justice and real democracy - stated that they intend to occupy the building - their fifth occupation - until such time as the City of London Corporation publishes full details of its City cash accounts.
"The City of London Police have visited the building and have agreed that it is a civil matter."
A Berkeley Homes spokesman said: "We urge the protesters to vacate this building site as quickly as possible, as we are very concerned that they are putting both themselves, and members of the public, in real danger.
"It is not safe for public use, there are holes in the floors and we are in the early stages of asbestos removal.
"The protesters are misguided in their actions which are sadly preventing Berkeley Homes from implementing their planning permission and so providing not only 90 much-needed new homes, but also a significant number of key construction jobs during an economic crisis.
"We are taking legal advice, particularly given the safety concerns, to ensure this potentially dangerous occupation ends quickly."
Occupy supporter Bryn Phillips, 28, dismissed the safety concerns, saying: "There was a log book inside the building and it was completely blank for asbestos.
"And we're only going to be occupying one floor at a time and that would be health and safety checked first."
The protesters plan to open the building to the public on 23 January, which will be the 100th day since they began the London chapter of a global movement against "corporate greed".
The group said it would use the venue to hold lectures and events, beginning with a lecture on the City's "secret finances and lobbying activities".