Police detained the treasurer of the Convergence party, Andreu Viloca, along with at least three businessmen in the province of Tarragona as part of an ongoing corruption probe, a police spokesman said.
A total of 10 arrests were expected as part of the operation which involves searches of about 20 homes and offices, she added.
The arrest came just hours before Artur Mas head of the regional Catalan government, was to summon a new session of parliament following last month's election in Catalonia.
His Together for Yes coalition clinched 62 seats during the September 27th polls, six seats shy of a majority in the 135-seat Catalan parliament.
The police operation took place as Mas seeks to garner support from leftist Popular Unity Candidacy (CUP), whose 10 seats would give pro-secession parties an outright majority in the assembly.
During the campaign, Mas had promised that a victory for separatist parties would mean independence for Catalonia – a region home to 7.6 million people – within 18 months.
While the CUP backs secession, it has criticised Mas' support for belt-tightening measures and corruption scandals affecting his party.
The arrest is part of a corruption probe that began last year with the detention of former Convergence mayor of the Catalan town of El Vendrell who is suspected of receiving bribes in exchange for public works contracts.
At the end of August, police raided the Barcelona headquarters of the Convergence party and its CatDem Foundation, as well as the headquarters of former party treasurer Daniel Osacar, just as the regional election campaign was about to officially kick off.
Spanish authorities suspect the foundation collected commissions from companies that won public works from town halls that were run by Mas' Convergence party.
The party was already under investigation for alleged illegal financing as part of another corruption scandal that erupted in 2009.
The founder of Convergence, Jordi Pujol, who ruled Catalonia from 1980 to 2003, resigned all his honorary positions last year after admitting he kept an undeclared fortune in tax havens for over three decades.