None of the paintings had been recovered and the investigation was continuing, police added.
Art specialists alerted police
In February, a British art firm contacted investigators after receiving a request to verify the authenticity of some art.
The person who contacted the firm lived in the northern Spanish city of Sitges, police said. They had included photographs of canvases purporting to be by Bacon. The person asked the experts if the works were listed as stolen.
Investigators then analyzed the photo and were able to determine that the camera that took the images was owned by a camera rental firm which supplied details of the customer who had rented it at the time the paintings were photographed.
`Highly professional` theft
The customer, who is suspected of involvement in the crime, was among those arrested, along with a Madrid art dealer and his son.
The other suspects also received the photographs and were arrested on suspicion of being accomplices and of conspiring to conceal the facts, police said.
Sources close to the investigation said in March that the theft appeared to have been a highly-professional operation which took place while the owner was away in London, with the perpetrators disabling the alarm system.
Irish-born Bacon died in Madrid in 1992, aged 82 and his expressionist-surrealist works, which are often raw and emotional, remain hugely sought after.
In 2013, his 1969 work "Three Studies of Lucien Freud" fetched 128 million euros ($142 million) at auction, a world record at the time.
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