POLICE in Spain have arrested four people, believed to be pro-Catalan independence campaigners, amid allegations they attempted to sabotage one of the cycling world tour's largest events.

The third stage on La Vuelta a Espana, which runs from the Catalan capital of Barcelona to the Pyrenees mountains, was the alleged target.

Spain's national police force said there was a plan to empty two drums containing 400 litres of liquid, believed to be similar to motor oil, onto the road as cyclists passed.

However, a discovery on a bridge along the route on Saturday led to the arrests.

A video from the police force and posted on social media showed a black tube running from hidden barrels over the edge of a bridge.

“With this mechanism they intended to sabotage yesterday’s stage of La Vuelta,” said a statement posted on Twitter/X in Spanish.

“Two drums with 400 litres of liquid that they intended to throw on the road as the peloton passed.

"Four people arrested red-handed in Lleida.”

The video also shows two pairs of people being led by police into a large building.

Officers said they believe the people are members of a pro-Catalan independence group.