MADRID (AP) — Taxi drivers across Spain's are striking to protest the increase in cars run by private companies offering cheaper, mobile ride-hailing services.

Travelers arriving at Madrid's or Barcelona's airports Tuesday found none of the usual lines of taxis waiting to pick them up.

The strike was directed against the San Francisco-headquartered Uber and Madrid-based Cabify services.

Unions claim these companies flout a law that stipulates there should be one private company vehicle for every 30 taxis.

Uber said it respected the strike but added that the sector needed to modernize.

The strike was to last from 6 a.m. to 6 p.m. in Madrid while in Barcelona it was to end at midnight.

Taxi drivers are to stage a demonstration in Madrid at noon.