Mexico City authorities are banning more than one million cars from the roads and offering free public transport as the first air pollution alert in 11 years stretches into a third day.

Officials are advising residents to limit outdoor activity due to high ozone levels that are nearly double acceptable limits.

Environment Secretary Alejandro Pacchiano said that if conditions do not improve, further measures may be considered - such as suspending industrial activity at some factories.

Mexico City Mayor Miguel Angel Mancera told Televisa news that about 1.1 million cars in the capital and surrounding valley have been ordered off the streets under the restrictions.