Four men arrested Saturday had planned to release 400 litres (88 gallons) of "a liquid similar to motor oil" onto the road apparently as part of a pro-Catalan independence protest.

The third Grand Tour of the season has been hit with problems in the opening stages, from bad weather to reigning champion Remco Evenepoel crashing into a spectator after the finish line on Monday as he won stage three.

UAE Team Emirates' Colombian rider Molano launched first in Tarragona and looked set to triumph but Australian sprinter Groves, riding for Alpecin-Deceuninck, roared past him to clinch the stage victory.

"I thought (I could win) because he had led out for 350 metres and a finish like that is super tough, I thought he would have to be on a really good day to beat me," Groves told Eurosport.

"It's been a good year, with a stage win in the Giro and already winning in the Vuelta quite early, hopefully it's not the last."

General classification leader Evenepoel keeps the red jersey and holds a five second lead on second place Enric Mas, with back-to-back Tour de France winner Jonas Vingegaard fourth, 31 seconds behind.

The relatively flat first section, starting from Andorra and heading back towards Catalonia's seaside, allowed a quick pace to be set with the peloton not allowing the breakaway of Eduardo Sepulveda, David Gonzalez and Ander Okamika off the leash.

The break survived both of stage four's two big climbs but they were caught by the peloton with under 20 kilometres remaining.

In the middle of the bunch several riders crashed, including Bryan Coquard and Santiago Buitrago, four kilometres from the finish, while EF Education-EasyPost rider Marijn van den Berg fell on a tight final corner.

Stage five on Wednesday takes riders 186.5 kilometres from Morella to Burriana, with another sprint finish expected and just one mountain challenge.

This is the 78th edition of the race, which ends in Madrid on September 17th after 21 stages and 3,153.8 kilometres.