Emergency repairs are underway after cracks were discovered beneath New York City subway tracks, officials say.

Emergency repairs are underway beneath Central Park West after transit workers discovered cracks in the trackbed during routine weekend maintenance, forcing major service changes on four subway lines Monday.

The Metropolitan Transportation Authority halted downtown local service on the A, B, C and D lines between 125th Street and 59th Street-Columbus Circle.

Repairs are taking place at 81st Street and Central Park West, where workers found the damage beneath the tracks.

Downtown B and C trains are running express through the affected area, bypassing all local stations. The MTA suspended B train service entirely in the Bronx and Upper Manhattan through the end of Monday.

Uptown service on all four lines continues with delays, though B and C trains are making their regular local stops in that direction.

The MTA said crews are working continuously to install a temporary fix and restore normal service. The agency plans to schedule permanent repairs later but has not announced when the emergency work will be finished.

The disruptions affect thousands of commuters who rely on the busy transit corridor through the Upper West Side.