Spain's Guardia Civil police on Wednesday searched several headquarters in Barcelona of Catalonia's regional government, a Catalan government spokesman said.

The operation comes amid mounting tensions as Catalan leaders press ahead with preparations for an independence referendum on October 1 despite Madrid's ban and a court ruling deeming it illegal.

Police were searching the Catalan government's offices of economic affairs, foreign relations and the presidency, the spokesman said.

The operation comes a day after police seized a trove of documents related to the independence referendum from the offices of private delivery company Unipost in the Catalan city of Terrasa.

Pro-separatist parties captured 47.6 percent of the vote in a September 2015 regional election in Catalonia billed as a proxy vote on independence, giving them a narrow majority of 72 seats in the 135-seat Catalan parliament.

But polls show Catalonia's roughly 7.5 million residents are divided on independence.

A survey commissioned by the regional government in July showed 49.4 percent of Catalans were against independence while 41.1 percent were in favour.

Over 70 percent of Catalans want a legal referendum on independence to settle the issue.