Mexico's president demanded on Thursday stricter vehicle emissions controls for the country's sprawling capital as an air pollution alert prompted authorities to restrict traffic for a third straight day.

Some 1.1 million vehicles were banned from the metropolis, children and the elderly were encouraged to stay indoors, and bus and subway services were offered for free amid the first high ozone alert in 14 years.

President Enrique Pena Nieto, meanwhile, instructed the environment ministry to establish "as soon as possible" a new vehicle emissions test system with the best available technology to "ensure low emissions of pollutants" from cars.

"But we can't leave it at that. We need to go further, be more audacious and more determined" to prevent air pollution, he said in a speech at the inauguration of a military health center.

Pena Nieto said the environment minister will have to coordinate with authorities in the capital and five surrounding states to create the new rules for a metropolitan area of 21 million people, with five million vehicles out on the road each day.

The pollution alert marks a reversal from years of progress to improve air quality after the United Nations declared the Mexican capital the world's most polluted city in the 1990s.

The air quality alert was issued after ozone concentration surpassed the 190-point limit, surging to 194, which can cause respiratory and cardiovascular ailments.

City authorities say a court ruling last year watered down a program that limits the number of days that older cars can be on the road, causing one million more vehicles to swarm the streets.