Economic justice campaigners who have been camped outside St Paul's Cathedral for weeks announced today that they have occupied an abandoned office block owned by investment bank UBS.

The occupation of the block in Hackney, east London, is the third stunt pulled by the Occupy London group - and the first time they have occupied a building.

They have previously targeted the land around St Paul's near the London Stock Exchange and Finsbury Square in Islington, north London, for their protests.

The building they have taken over at Crown Place belongs to, but is not occupied by, UBS and no business transactions take place there.

The activists plan to set up a "bank of ideas" there tomorrow and open the disused offices and meeting rooms to "those who have lost their nurseries, community centres and youth clubs due to savage Government spending cuts".

A programme of events has been drawn up, including talks from Palestinian activists and comedy by Josie Long, they said.

They described the move as a "public repossession", which they carried out overnight last night when a dozen of them gained access and secured the building.

They now had a "legal claim" on the space, they said.

Occupy London supporter Jack Holburn said: "While over 9,000 families were kicked out of their homes in the last three months for failing to keep up mortgage payments - mostly due to the recession caused by the banks - UBS and other financial giants are sitting on massive abandoned properties.

"As banks repossess families' homes, empty bank property needs to be repossessed by the public.

"Yesterday we learned that the Government has failed to create public value out of banking failure. We can do better. We hope this is the first in a wave of 'public repossessions' of property belonging to the companies that crashed the global economy."

The occupation will not resemble the St Paul's Cathedral camp as visitors are asked not to bring sleeping bags because the "bank of ideas" is a non-residential occupation.

The group said drugs and alcohol would be banned as well "as per Occupy London's safer space policy".

Activist Sarah Layler said: "The bank of ideas will host a full events programme where people will be able to trade in creativity rather than cash.

"We will also make space available for those that have lost their nurseries, community centres and youth clubs to savage Government spending cuts."

A UBS spokeswoman said: "We are aware of the situation and are taking appropriate action."