Spain's Guardia Civil police force on Wednesday arrested the Catalan vice president's right-hand man, police and Catalonia's regional government said.
The arrest of Josep Maria Jove, the secretary general of the Catalan vice presidency, comes amid mounting tensions as Catalan leaders press ahead with preparations for an independence referendum on October 1st 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.
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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.