Kaden Groves won a thrilling sprint finish to beat Sebastian Molano and win stage four of the Vuelta a Espana.
Four people were arrested for planning to spill hundreds of litres of a liquid similar to motor oil on the road.
They are thought to be part of a group that is seeking the independence of Catalonia and were later released, pending further investigation.
The incident is the latest in a number of issues to have plagued the start of the Grand Tour.
The opening two stages were affected by torrential rain, while defending champion Remco Evenepoel suffered a cut above his right eye which bled profusely after crashing into a fan after winning stage three on Monday.
The fourth stage was a 184.6km ride from Andorra la Vella to Tarragona which included two category three climbs in the final 60km.
After the breakaway of Eduardo Sepulveda, Ander Okamika and David Gonzalez were caught by the peloton with 19km to go a sprint was inevitable.
Colombia’s Molano launched first and looked set to triumph before 24-year-old Groves, riding for Alpecin-Deceuninck, sped past him to clinch victory on a difficult finish.
“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,” said Groves.
“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.”
Belgium’s Evenepoel keeps the red jersey and holds a five-second lead over second-placed Enric Mas in the general classification.
Stage four results
1. Kaden Groves (Aus/Alpecin-Deceuninck) 4hrs 5mins 31secs
2. Juan Sebastian Molano (Col/UAE Team Emirates) Same time
3. Edward Theuns (Bel/Lidl-Trek)
4. Milan Menten (Bel/Lotto Dstny)
5. Dries van Gestel (Bel/Totalenergies)
6. Orluis Alberto Aular Sanabria (Ven/Caja Rural-Segurosrga)
7. Hugo Page (Fra/Intermarche – Circus – Wanty)
8. Lewis Askey (GB/Groupama-FDJ)
9. Sean Flynn (GB/Team DSM-Firmenich)
10. Andrea Vendrame (Ita/AG2R Citroen Team)
General classification after stage four
1. Remco Evenepoel (Bel/SoudalQuickStep) 12hrs 48mins 52secs
2. Enric Mas (Spa/Movistar Team) +5secs
3. Lenny Martinez (Fra/Groupama – FDJ) +11secs
4. Jonas Vingegaard (Den/Jumbo-Visma) +31secs
5. Alexsandr Vlasov (Rus/Bora – Hansgrohe) +33secs
6. Cian Uijtdebroeks (Bel/Bora – Hansgrohe) Same time
7. Romain Bardet (Fra/Team DSM-Firmenich) +35secs
8. Primoz Roglic (Slo/Jumbo-Visma) +37secs
9. Juan Ayuso (Spa/UAE Team Emirates) +38secs
10. Marc Soler (Spa/UAE Team Emirates) +42secs
