GENOA, Italy — A bridge in the heart of Genoa collapsed on Tuesday, killing 37 people as dozens of vehicles and tons of concrete and steel plunged onto city streets below in a disaster that prompted an emergency review of Italy’s aging infrastructure.

Adriano Scimpioni said he had just finished his shift at the city’s garbage collection company when “I heard something like an explosion and a screech of iron, and in a second we were all covered by a cloud of dust.”

Mr. Scimpioni went outside, and with his smartphone he started broadcasting the scene on Facebook.

“I immediately thought of the victims; I held my tears, with difficulty,” he said. “We were impotent, we didn’t have the means to help those who remained underneath.” He said two of his colleagues had been killed.