Two pipe bombs apparently planted by Puerto Rican terrorists exploded tonight in a Penn Station waiting room jammed with holiday travelers, sending thousands of people into the frigid streets and delaying train service.

No injuries were reported, but one police officer said, "It's a wonder no one was killed."

Authorities found fragments of two "quite powerful" pipe bombs in lockers in a central lobby area, Police Chief Milton Schwartz said. The blasts, police and ambulance officials said, occurred about 10 minutes apart, before and after 6 p.m.

Dogs trained to sniff out explosives were used to search other lockers.

The multilevel station, bounded by Seventh and Eighth avenues and 31st and 33rd streets, is used daily by thousands of Amtrak, Conrail and Long Island Rail Road passengers.

Damage was limited to a bank of lockers between a stairway and a small restaurant catering to commuters.

Police said the bombs apparently had timing devices and were each as strong as "one stick of dynamite."

A man who telephoned United Press International said he represented the Puerto Rican Armed Resistance and "we take full responsibility for the bombing tonight."

The caller gave directions to a communique and police said a one-page message typewritten in Spanish was found in a garbage can a couple blocks away.

He said the group "alleged that the bomb was set in retaliation for crimes against Puerto Rican nationalists in Puerto Rico."

"I was near the information desk [about 100 feet away] when I heard two 'pows.' I thought someone was shooting," said Dianna James of Huntsville, Ala. "I'm here for Christmas and I'm going home real soon and I'm not coming back," she added.

The station was closed until 8:04 p.m.

Until then, a Long Island Rail Road spokesman said its trains were terminated at Woodside in Queens, where passengers could get subway service into Manhattan. Amtrak said three westbound and seven eastbound trains were delayed from entering the station.