MEXICO CITY—At least 24 people died and dozens more were injured when an elevated section on the Mexico City subway collapsed in the southeast of the country’s capital, causing several train cars to plunge into the street below.

Mexico City officials said 27 people were hospitalized as of Tuesday afternoon, several of them in serious condition.

The collapse occurred late Monday on Mexico City Metro’s Line 12, which opened in late 2012 and is the newest line in the system. The city’s subway system is the second-busiest in the hemisphere after New York and transports some 4.5 million people a day. Part of the line is underground, and another section runs along an elevated rail.

Mexico City Mayor Claudia Sheinbaum said a support beam holding up the railway collapsed as the subway passed over it, leaving the train split in two and hanging precariously. She said authorities are investigating why the beam buckled and fell.

Mexican President Andrés Manuel López Obrador promised a full investigation into the rail disaster, which killed several minors.