The Catalan government earlier this week accused the Spanish authorities of effectively taking over its administration after state police raided its offices and arrested officials, and the finance ministry took control of the local finance department.
Catalan Vice-President Oriol Junqueras acknowledged that the crackdown had disrupted the referendum plans, saying, "It's evident that we won't be able to vote as we have done in the past".
Spain's Europa Press news agency and other media outlets said the raids Wednesday mostly targeted the region's economic and foreign departments as Spanish authorities worked to halt all preparatory moves for the planned October 1referendum.
An Interior Ministry statement said the extra police will provide backing for Catalan regional police who are also under orders to prevent the staging of the referendum.
The reinforcements will join 5,000 state police already based in the region. A statement said the six declined to testify.More news: Julia Louis-Dreyfus Breaks Record For Emmy Wins
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After the Barcelona attack, in which a young man drove a van through a crowded street, police discovered the terrorist cell had been planning a much more serious attack.
Hundreds of pro-independence supporters had protested outside the courthouse to demand the officials' release.
They remain under investigation for disobedience, abuse of power and embezzlement in relation to the planned referendum and were ordered to appear before the court every week.
Catalonia represents a fifth of Spain's 1.1 trillion-euro (£970 billion) economy and enjoys wide self-government. The national interior ministry's press office didn't respond to multiple requests for comment.
The region has about 5.5 million eligible voters, and polls consistently show the region's inhabitants favour a referendum but are roughly evenly divided over independence from Spain.
However, neither the Catalan government nor the Catalan police force will accept the new leadership, according to Catalonia regional government's interior minister, Joaquim Forn.