The government in Madrid is facing one of its biggest political crises since the end of the Franco dictatorship and the return of democracy four decades ago.
An Interior Ministry statement said the extra police would provide backing for the Catalan regional police, who are also under orders to prevent the staging of the referendum.More news: Liverpool: If Danny Ward excels he should play in the Premier League
While Mossos chief Josep Lluis Trapero reports to the regional government, his force's funding is mostly provided by Madrid and it's supposed to take orders from judges and prosecutors from across the country.
"All you need is eyes in your head to see there is no violence", spokesman Jordi Turull said.
The arrests were the first involving Catalan officials since the campaign to hold an independence vote began in earnest in 2011.
State police already on the ground carried out unprecedented raids on regional government offices, printers, newspapers and delivery companies, searching for campaign material on Wednesday.More news: Mahira, Ranbir pictures become target of fans' criticism
The Spanish resident, originally from Morocco, had a close relationship with those responsible for the attacks in Barcelona and Cambrils which left 16 dead, according to a statement from Spain's Interior Ministry. In addition, the finance ministry seized control of the local finance department amid fears that Catalan officials were using Spanish tax dollars to finance the ballot. "It's a clear attempt to discredit it", said Joaquim Forn on Twitter.
The uneasy power-sharing accord that the Mossos have had with the Civil Guards for decades frayed further this week when a union of Civil Guard senior officers issued a statement calling on Rajoy's government to take sole charge of the Catalan police.
Some are billeted in three ferries rented by the Spanish government and moored in the harbours of Barcelona and Tarragona because of both a shortage of temporary housing and to avoid clashes between protesters and police forces.More news: Flacco, aggressive defense lift Ravens past Browns