NEW YORK (Reuters) - A unexpectedly strong blast at an underground construction site for New York's $4.5 billion subway expansion startled people on Manhattan's East Side on Tuesday, shattering windows, damaging a building and forcing a street closure, officials said.

"I heard a loud explosion like a big boom and it shook the ground," said Edwin Malabe, 54, a doorman at a building on East 74th street.

The blast at East 72nd Street and Second Avenue caused minor damage, transit officials said. It blew out windows and sent people including construction workers scattering onto the streets.

Firefighters rushed to the scene, though no injuries were reported. Officials temporarily closed 72nd Street.

"There were stones everywhere, a lot of cement, dust, and broken glass. I was nervous. I was really worried about my co-workers at the store," said Diana Mighiu, 36, who arrived for work at the Kolb Art Gallery about half an hour after the blast.

"All work at the 72nd Street site is suspended until we establish cause and the contractor can guarantee it won't happen again," said Kevin Ortiz, a spokesman for the Metropolitan Transportation Authority.

Destroyed power lines fell onto a closed sidewalk, knocking down a fence set up at the construction site.

The Second Avenue subway line is planned to run up and down the east side of Manhattan.

"We were doing a controlled blast when clearly something went awry and an explosion was felt at street level," Adam Lisberg, another MTA spokesman, told The New York Times.

(Reporting by Lily Kuo)