Activists from the Occupy London movement have abruptly changed tactics, following a month of open-air tent protests, by seizing a vacant office complex owned by a major global bank.

In the early hours of Friday morning, protesters entered a series of interconnected buildings owned by UBS on the fringe of the City, which have lain empty for several years. They immediately claimed squatters' rights, posting a legal notice to that effect.

By mid-morning, several dozen people were inside, cleaning up patches of damp and collapsed ceiling plaster and hanging giant banners from windows, several targeting the actions of UBS, which has its London headquarters are across the street.

In contrast to Occupy's existing camps in the capital – next to St Paul's Cathedral and on Finsbury Square – the new outpost will not be primarily residential but aims to act as a forum for ideas and meetings.

Called by protesters the Bank of Ideas, the buildings, near Liverpool Street, will hold an inaugural conference of Occupy movements from around the UK this weekend. A statement by the group promised a show by the comedian Josie Long and a seminar by Alessio Rastani, the independent financial trader who made headlines in September by telling the BBC that investment banks were now more powerful than governments.

"The Bank of Ideas will host a full events programme where people will be able to trade in creativity rather than cash," said Sarah Layler of the group. "We will also make space available for those that have lost their nurseries, community centres and youth clubs to savage government spending cuts."

It marks a further sign of the growing confidence of the UK movement, which sprang from an international wave of protests against the perceived excesses and inequalities of the global financial system.

Occupy London has attracted significant attention since opening its camp on land next to St Paul's on 15 October after police thwarted an initial plan for a base next to the London Stock Exchange. A fierce debate was sparked within the Church of England about equality and the right to protest, resulting in two senior clergy leaving their jobs.

Unlike their counterparts in the US, who have faced a number of forcible police evictions, Occupy protesters in London have been left alone, in part because of the church's decision to tolerate the St Paul's camp.

An attempt by the Corporation of London to have the St Paul's camp removed is set to reach the high court on Wednesday. The Finsbury Square camp is on land owned by Islington council, which has privately told activists it does not have the money to pursue a court case.

UBS, however, could apply very quickly for legal repossession of the buildings and request bailiffs, backed up by police, to move in. A spokeswoman for the Swiss-based bank said: "We know they're there, and we're taking appropriate action."

A building in an adjoining street was occupied briefly by anti-globalisation protesters ahead of the G20 meeting in London in April 2009. Within days, it was stormed by police in a raid which, activists alleged, involved the use of stun guns.

At a hastily-arranged press conference inside the UBS building, about a dozen activists answered questions while sitting on a stained blue office carpet in front of a vast Occupy London sign. The group said it hoped to avoid confrontation with police and had asked UBS about agreeing temporary use of the site. UBS said it had no comment on this offer.

Aside from the building's size and location, near many other financial institutions, the group said it was chosen as it is owned by a major bank, and one which was bailed out by taxpayers, albeit those in Switzerland. Ronan McNern, a spokesman, said: "UBS is representative of the sort of bank which is not acting in the public interest. This is a public repossession of their empty building."

Another symbolic element of the site is that, while close to many financial institutions, it lies just outside the area administered by the very wealthy – and, activists claim, highly unrepresentative – Corporation of London, in Hackney, one of the UK's most deprived boroughs.

The occupation has been assisted by veteran squatters' rights campaigners, who hope to use the site to highlight an imminent new law that will make the practice a criminal offence. Currently, squatters must be evicted via the civil courts as long as a building is empty and there is no damage caused in gaining entry.

One campaigner, environmental consultant Pete Phoenix, 41, showed the Guardian round several dozen of the several hundred empty rooms in the complex. This vast size and emptiness served as a symbol of poor use of resources, he said. "This winter there's people freezing on the streets, and look at all of this wasted space."

There is a slight potential conflict between the protesters' stated intention to make the building a public space and its status as a squatted building. This will be overcome by designating visitors "friends" or "guests".

In the interim, the building remains shut to outsiders, making it a different prospect to the open-to-all ethos of the other camps, where passersby and local workers have been welcomed.

There seemed little imminent prospect of detente with UBS staff: soon after the first Occupy banners were hung, UBS workers on the other side of the street pulled down all the blinds in their offices.