Thousands of drivers for ride-hailing services Uber and Cabify, waving flags and chanting slogans such as "we want to work", marched down Madrid's central boulevard on Thursday ahead of plans by the government to tighten regulation.
Drivers from both companies, which have faced complaints from taxi drivers all over the world for allegedly providing unfair competition, offered free rides for 12 hours a day before the strike, business association Unauto said.
In Spain, there are almost 11,000 vehicles with ride-sharing licenses and more than 65,000 with taxi licenses, Public Works Ministry data shows, with more than 150,000 taxi drivers and 15,000 Uber and Cabify drivers operating throughout the country.
Cabify driver Juan Antonio Sastre, 57, who left the ranks of millions of long-term unemployed to join the company, said he feared for his job ahead of the new regulation to be passed on Friday.
"We don't know what is going to happen next, our future is uncertain," he said, while taking part in the march.
Full details of the new regulation have yet to be announced, though new laws are expected to include additional restrictions for non-taxi services offered by the companies.
Backed by investors including Goldman Sachs and BlackRock, and valued at more than $70 billion, Uber views Western Europe as an increasingly important market.
Uber has faced law suits in countries around the world, with London cab drivers planning a class action law suit and New York mulling capping the services after a spate of suicides by yellow cab drivers struggling to compete.
Taxi drivers, who staged their own six-day strike at the beginning of August to protest against licenses for Uber and Cabify, say the services, which are hailed via smartphone apps rather than on the street, are deliberately under charging.
"We cannot compete against corporations like Uber and Cabify, their prices are way too low," Madrid taxi driver Jorge Gordillo, 33, said.