Madrid has activated an anti-pollution order that significantly restricts private vehicles in the city centre, including a total ban on the most polluting cars.
The much-debated plan establishes a 472-hectare low-emission zone in the heart of the Spanish capital.
Petrol vehicles manufactured before 2000 and diesel ones registered prior to 2006 are banned from the area or will face a 90 euro (£80) fine.
Only public buses, taxis, residents and some professional vehicles are exempt.
Electric, zero-emission cars are allowed in.
Some local politicians say the plan will have meagre effects on air quality and is a publicity stunt by the city’s mayor Manuela Carmena.
Ms Carmena, a left-wing former judge, said those opposed to the changes will get used to the restrictions and increase their use of public transport.