A video has emerged online showing police stopping and boarding a bus, allegedly on its way to the mass anti-austerity demonstration in Madrid.
Other reports from Madrid report people being stopped as they arrive in the capital, either on buses or trains and asked for identification as well as having their bags searched.
The person who took the video turned off their device shortly after the police boarded the bus, it cannot be confirmed that a search or identification took place.
Thousands are expected in Spain's capital today to protest austerity measures which have left many people unemployed, and in poverty. Starting at 17:30 (15:30 GMT) demonstrators will surround the congress building in Madrid in what has been dubbed 'Occupy Congress'.