Mexico City, Mexico — Thousands of people marched and blocked roads in Mexico's capital to protest a new anti-pollution measure that would ban them from driving on Saturdays.

Mexico City has long banned older cars from operating one weekday every week, but anti-smog rules that recently took effect additionally now prohibit cars and trucks more than 15 years old from also operating on Saturdays.

The protesters marched through downtown and also blocked traffic on the main ring road serving the metropolis of 9 million holding placards that asked, "Driving is only for the rich?"

The new measure is particularly hard on vendors who sell fruit and vegetables at street markets. They often use creaky old trucks to haul their produce to markets set up on city streets on weekends, where they sell groceries at lower prices than in supermarkets.

Vendor Elvira Castro has sold vegetables for most of the last three decades at Mexico City street markets, most recently using an old 2000-model pickup to haul produce. While her truck could operate one more year under the new rules, it would be pulled off the road next year.

"They're leaving us without work," Castro said. "It's not fair."

The city's air quality has improved from the worst days of the early 1990's, when smog reached almost four times the pollution-alert level. In 2012 and 2013, pollution reached unacceptable levels on about one-third of days. But the growth in the number of newer cars purchased to beat the restrictions on older models has led the city to enact new measures.