A steam pipe explosion ripped open a street next to Grand Central Terminal yesterday, sending jets of stream and rocks into the air during the afternoon rush hour and killing one person.

More than 20 others were injured, some seriously, Mayor Bloomberg said, as glass and debris rained down on panicked crowds at about 6 p.m.

“There was a loud ‘vroom,’ and big rocks were shooting up in the air,” an air-conditioner repairman, Austin Dove, who was standing less than a block from the explosion at Lexington Avenue and East 41st Street, said. “People were running crazy. You had people trampling each other and running people down.”

Authorities quickly downplayed fears that the explosion was a terrorist attack.

“There is no reason whatsoever to believe that there is anything involved with terrorism or criminality,” Mr. Bloomberg said at a news conference near the scene that was punctuated by the sound of sirens. “It is probably just a failure on the part of our infrastructure.”

The mayor said a 20-inch steam pipe installed in 1924 had probably ruptured when cold water had come in contact with the pipes following a day of heavy rains and flooding. The chairman of the power company Con Edison, Kevin Burke, said inspectors had been looking for just that phenomenon at the site of the explosion earlier in the day. He said they had not found any telltale vapor that would have alerted them to a problem.

Authorities yesterday evacuated people from a “frozen zone” around the site to test for asbestos that may have been released during the explosion.

“The big fear that we have is that there may or may not have been asbestos,” Mr. Bloomberg said, dismissing speculation that other toxic substances could have been released.

The president of TransGas Energy Systems, Adam Victor, said there are 130 miles of steam lines — some more than 100 years old — running between 96th Street and the Battery in Manhattan that serve as the exclusive source of heat to 2,200 buildings, including every major hospital in that area. The one fatality yesterday was an unidentified person who died of cardiac arrest. At least four others were seriously injured.

More might have been hurt if not for the efforts of a passing police officer.

Before the explosion, people walking on the street noticed a rumbling and flagged down Officer Robert Mirfield, 42.

An NYPD spokesman, Deputy Commissioner Paul Browne, said Officer Mirfield put on his helmet and began evacuating a city bus. When the pipe blew, Officer Mirfield was catapulted backward onto his truck and injured his right arm.

Mr. Browne added: “When they found him he was assisting others.”

Many witnesses describing the mayhem said they couldn’t help but recall the terrorist attacks of 2001.

As buildings shook and a cloud of black smoke enveloped the crowds in the surrounding blocks, women running in high heels abandoned them in favor of bare feet, witnesses said. Those who emerged were covered in a dusting of rust-colored powder.

“It was like a mini volcano erupted,” a paralegal in a suit turned orange from the dust, Victor Montalvo, 36, said.

“We all thought it was terrorists again,” a 29-year-old Roosevelt Island resident, Ester Mbwale, who was injured in the blast, said. “I was about to cross the street and it exploded right in front of my face.”

East Side subways were diverted and the blocks between East 40th and 43rd streets and Vanderbilt and Third avenues cordoned off. Thousands of commuters were forced to trek home by foot.

By 9:30 p.m., Con Edison had shut down the vapor flowing through the pipe.