Catalonia, which wants to hold a vote on Oct. 1 to split from Spain, has its own police called Mossos d'Esquadra, though the state police Guardia Civil has quarters throughout the region and often works alongside them.
The interior ministry has not disclosed how many more police officers it is sending to the region but Spanish media said between 3,000 and 4,000 had already arrived or were on their way, Reuters reported.
They would join the roughly 5,000 state police normally stationed in Catalonia and around 17,000 Mossos officers.
"They will be tasked with surveillance of public space and maintaining order and they will act in case the illegal referendum is maintained," the interior ministry said in a statement.
They will act as a backup for the Mossos, who will remain in charge, it added.
Some are billeted in three ferries rented by the Spanish government and moored in the harbors of Barcelona and Tarragona because of both a shortage of temporary housing and to avoid clashes between protesters and police forces.
The interior ministry has told police in Catalonia they will not be able to take time off between Sept. 20 and Oct. 5.
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.
The Catalan regional leader Carles Puigdemont on Thursday said he would press on with the referendum, flouting a court ban, as tens of thousands gathered for a second day on the streets of Barcelona demanding the right to vote.