"Three ships arrived and will stay in the ports of Barcelona and Tarragona, where police and Guardia Civil forces will stay," said a source at the central government's representative office in Catalonia, who declined to be named.
Two of the ferries are moored in the port of Barcelona while another is in Tarragona, some 100 kilometres (60 miles) south of the Catalan capital. One of them is decorated with giant cartoon characters representing Wile E Coyote, Tweety-Pie and Daffy Duck.
A spokesman for the Barcelona port said they are due to stay until at least October 3rd, or two days after the planned referendum.
The three ships have a capacity for 6,600 passengers, giving an idea of the number of extra police sent to a region deeply divided over independence as authorities implement a series of measures to stop the vote from taking place.
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Police in riot gear at a protest in Barcelona on Wednesday. Photo: AFP
Police have seized close to 10 million ballot papers destined for the vote and over 45,000 notifications due to be sent to Catalans selected to staff polling stations.
Prosecutors have threatened to arrest mayors who facilitate the vote if they do not comply with a criminal probe and Madrid has tightened control over the region's finances.
They also arrested 14 key members of the team organising the vote on Wednesday, although eight have since been released.
The measures caused outrage among Catalan independence supporters, with several thousand taking to the streets of Barcelona.
As such, dockers in the ports of Barcelona and Tarragona said in a statement they would not attend to the ferries where police forces are staying.