The Occupy movement, under imminent threat of eviction from its encampment at St Paul's Cathedral, has expanded into a third central London location after more than 20 activists moved into an abandoned office block owned by the banking giant UBS.

Pledging to turn the four-storey building into a "Bank of Ideas", the group, which has established a presence at 30 locations in cities and towns around the UK, called the occupation a "public repossession" and plans an alternative community centre for those affected by government cuts.

Around 15 people entered the building early on Friday morning. Activists claimed that windows had been left open and no criminal damage had been caused. A spokesman for the Metropolitan Police said that officers were called to the property on Sun Street in London's financial district at around 3.45am, but left shortly after without recording a criminal offence.

The police maintained their hands-off approach yesterday, patrolling the streets in riot vans but not venturing into the building, while a spokeswoman for UBS said that the company would take "appropriate action" and did not rule out an eviction order.

Yesterday's occupation was the third such action in the capital. Activists set up tents outside St Paul's over a month ago and a second camp in Finsbury Square was established a week later.

Protesters said that, by occupying UBS property – which unlike most Occupy camps has running water, flushing toilets and kitchenettes – they were taking the public's rightful stake in banks' possessions.

"Whilst 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," said Jack Holburn, an Occupy supporter.

UBS, Switzerland's biggest bank, received a $60bn government bailout at the height of the financial crisis in 2008.

If allowed to stay, activists hope to open up the space for use by nurseries, community and youth groups.