In a private ceremony at the abandoned Old City Hall subway station, Mamdani was sworn in by New York Attorney General Letitia James as the clock struck midnight on New Year's Eve.
Dozens of people are presumed dead and about 100 others are injured after a fire tore through a bar at a Swiss Alps ski resort, police say. Authorities are working to determine the cause, but do not believe it was an attack.
Hundreds of thousands of people braved the cold weather in New York City to watch the iconic ball drop in Times Square on New Year's Eve. Meanwhile in London, fireworks lit up the sky to welcome 2026 and there were drone displays in the United Arab Emirates.
