New York City subway and bus riders who refuse to wear a mask while taking public transit will now be subject to a $50 fine, as part of an MTA crackdown aimed at achieving "universal mask compliance."

The new measure, which was announced last week, took effect on Monday morning. A spokesperson for the transit agency could not say whether anyone had been fined as of 3 p.m. afternoon.

"To be clear, this is not about revenue, and we have no interest in issuing fines or summonses," MTA CEO Pat Foye said during a news conference on Monday inside the Atlantic Ave-Barclays Center, where he and other transit officials spoke while not wearing masks. "This is about public health."

Governor Andrew Cuomo has pushed for harsher penalties against bare-faced riders, in an effort to encourage more New Yorkers to return to public transit. The rule will be enforced by both NYPD and MTA officers, as well as Bridge and Tunnel state police, according to transit officials.

Riders will be offered a mask before receiving a fine "in many cases," Foye said. The agency's volunteer-led "Mask Force" has already passed out 3 million masks to subway and bus riders.

Since the start of the pandemic, New Yorkers have continuously taken note of the NYPD's refusal to wear face coverings — during protests, interactions with suspects, and even packed indoor retirement ceremonies.

Speaking with NY1 on Monday, Police Commissioner Dermot Shea suggested that cops who don't wear masks while patrolling the transit system could themselves face a fine.

"The same rules would apply. Officers are not immune," Shea said. "We have internal policies that we can institute discipline if that's warranted."

The NYPD was previously removed from most social distancing enforcement this past spring, after a video showed officers violently arresting a woman for not having a mask in front of her young child in the Atlantic Ave-Barclays station.

Many transit advocates feared that resuming the cops' role in mask enforcement will inevitably lead to racial disparities — something the city has seen for years in all other aspects of subway policing.

Asked about this on Monday, Foye said he did not "accept the premise of the question."

Danny Pearlstein of the Riders Alliance called the fine a mistake, noting that mask compliance was already very high among among transit riders — more than 90 percent on both subways and buses, according to transit surveys.

"Rather than making transit safer," he said, "this will raise tensions and increase negative interactions between police and riders."