Each of the ferries has a capacity of over 6,500 passengers.
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.
Tension has been rising between Spanish and Catalan authorities, and Civil Guard police this week arrested around a dozen regional government officials and seized about 10 million ballot papers.
Catalonia's executive admitted Thursday that plans for an outlawed independence referendum had been dealt a major blow by a police crackdown, but vowed to press ahead, urging support from the global community.
Spain's Interior Ministry says it's deploying police reinforcements to its northeastern region of Catalonia to maintain order and take action if a referendum on independence pledged by the Catalan government but deemed illegal by Spain should take place.
The statement said the Catalan Interior Ministry had been informed.More news: 18-year-old charged with attempted murder in London tube terror attack
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Backed by most Spanish opposition parties, the government says the referendum violates the constitution and that if Catalonia wants a vote it must work to change the constitution first.
Catalonia represents a fifth of Spain's 1.1 trillion-euro (£970 billion) economy and enjoys wide self-government.
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.
Spain's Europa Press news agency and other media outlets said the raids mostly targeted Catalonia's economic and foreign departments as authorities worked to halt all preparatory moves for the referendum planned for October 1.
Student union representatives are calling for protesters to remain over the weekend to support for the vote.
A pro-independence group said about 2,500 supporters were at the protest in Hospitalet. The Catalan National Assembly, a driving force behind the secession movement, urged people to gather at noon September 21 outside the region's justice tribunal and bring tents if needed.