Four people have been arrested in the Spanish northeastern region of Catalonia for an alleged attempt to sabotage a stage of the Vuelta a Espana cycling race, Spanish Police said on Tuesday.

Those arrested were trying to spill some 400 litres of a liquid similar to motor oil on the road during Monday's third stage, which was 158.5-km ride between the towns of Suria near Barcelona and Arinsal in Andorra, police said in a statement.

A police source said they were members of a group known as CDR that is seeking the independence of Catalonia.

They had two drums, each with a capacity of 200 litres, plus an activation device with an electrovalve and timer hidden among bushes on a bridge over the road where cyclists were expected.

A court set them free pending further investigation but ordered them to stay at least 500 metres away from the cycling event.

They are being investigated for crimes against road safety, the environment, being part of a criminal group and public disorder.

Defending champion Remco Evenepoel won Monday's third stage, beating Jonas Vingegaard and Juan Ayuso to the finish line as he also claimed the leader's red jersey.

The three-week Spanish cycling race started in Barcelona on Saturday and is due to wind through Catalonia and Andorra for four days before crossing into the rest of the country.

The Vuelta finishes in Madrid on September 17.