Police mounted an intense manhunt on Tuesday for a gunman who set off two smoke bombs and opened fire in a New York subway car, injuring more than 20 people in a morning rush-hour attack that prompted new calls to fight violence in the city's transit system.

Police said the gunman was believed to have acted alone and immediately fled the crime scene. The attack unfolded as a Manhattan-bound subway train on the N line was pulling into an underground station in Brooklyn's Sunset Park neighbourhood...