Fares on the Mexico City subway jumped 66.7 percent after remaining unchanged for years, sparking protests in a least a dozen stations, authorities said.

In a message sent on Twitter, subway operator STC reminded the system's 4 million riders "that starting today the cost of a ticket is 5 pesos (39 cents), up from 3 pesos (23 cents)."

The measure was taken after the majority of citizens said in a survey taken weeks before that they would agree to an increase.

STC chief Joel Ortega said the subway functioned "with no significant problems" on Friday, though "with some protests in 12 stations."

A group of young people called on commuters to protest against the increase and enter the subway without paying for a ticket by jumping over the turnstyles, while at the same time calling for a march through the capital's downtown area.

In several subway stations, young people have taken control of the entrances so passengers can go through without paying and, waving their banners, they distribute leaflets explaining why they consider the fare increase unjust and question the validity of the citizen survey.

The STC said that with the new fares, it will begin to fulfill its commitments to improve the service by modernizing the system, purchasing new trains and eliminating the informal economy from the stations.

Also taking effect today is a subsidy for single mothers and the unemployed, who will have a special fare card with which they can continue paying 3 pesos.

More than 1,500 discount cards have been distributed, Mexico City Mayor Miguel Angel Mancera said on Twitter. EFE