The Occupy movement continued to acquire momentum on Saturday as protesters from camps across the country converged in London to begin shaping a national campaign.

The supporters – from more than 10 Occupy sites, including Plymouth, the Isle of Wight and Edinburgh – gathered as the campaign opened an empty office building owned by the Swiss bank UBS as a venue for discussions, after taking control of it on Friday. Christened the "Bank of Ideas", the vast complex on the periphery of the City is the third London site to be occupied, following encampments at St Paul's Cathedral and Finsbury Square. Organisers said that more sites would follow as the movement grew.

The fashion designer Vivienne Westwood, 70, became the latest high-profile supporter to address protesters on Saturday, on the steps of St Paul's. She said that the global financial crisis was intrinsically linked to the world's ecological travails, and called for people to embrace culture as a means to help wean them "off the drug of consumerism".

Her appearance followed a speech from Alessio Rastani, an independent financial trader, who has warned that "the savings of millions of people are going to vanish" and that investment banks had become more powerful than governments. His warnings were followed by an address from Nicholas Shaxson, the author of Treasure Islands, an investigation into tax havens and offshore banking networks. Occupy's critique of the financial system and its calls for a fairer replacement will continue next week with speeches at the TUC Conference Hall in central London, a sign that unions may be starting to form official alliances with the movement.

Unions are finalising plans for a day of industrial action against public-sector pensions cuts on 30 November.

Tanya Paton of Occupy London said: "Groups are coming together for the first time; the movement is becoming stronger throughout the UK. We are sharing ways to overcome problems and work together to build and define a national campaign."

Among other camps represented yesterday were supporters from Newcastle upon Tyne, Bath, Brighton, Bristol, Liverpool, Norwich and Cardiff. In the Welsh capital, police attempted to break up the protest. Occupy protesters in London, however, been left alone by police, unlike their US counterparts, who have endured forcible evictions. The move stems from St Paul's decision to tolerate the camp, which on Saturday celebrated its fifth week. Efforts by the Corporation of London to have it removed are expected to reach the high court on Wednesday. Swiss-based bank UBS could also try to reclaim its building by applying for repossession. Sources at the bank have pledged to take "appropriate action".

Visitors to the building on Saturday were designated "friends" or "guests" to help reconcile its status as a squatted building, and the protesters' aim was to make the building a public space. The new site will be opened and available for those that have lost community centres and youth clubs due to government spending cuts.