Heiko Thieme, a banker, said the explosion was like a volcano.
"Everybody was a bit confused, everybody obviously thought of 9/11," he said.
Michael Bloomberg, the city's mayor, said at the scene of the blast: "There is no reason to believe whatsoever that this is anything other than a failure of our infrastructure."
Subway service was suspended because of the explosion.
Besides the immediate chaos, there were also concerns about what was being spewed into the air.
A steam pipe explosion near Gramercy park in 1989 that killed three people spewed loads of asbestos into the air - a fact that the Con Edison utility company later admitted it concealed for days while residents were exposed.
Con Edison workers were at the scene, as were police wearing gas masks.