NEW YORK - A teen-ager opened fire in the hallway at a high school near Lincoln Center yesterday, seriously wounding two fellow students in what may have been a gang-related shooting, authorities say.

The gunman was arrested two blocks away, police said.

The shooting on Manhattan's Upper West Side occurred at Martin Luther King Jr. High School on what would have been the 73rd birthday of the apostle of nonviolence. The public school has 3,000 students.

Authorities did not immediately give a motive, but schools Chancellor Harold Levy said the shooting may have been gang-related. He said the suspect was an 18-year-old who had not been attending school.

"We were in school and we heard two gunshots," said senior Romain Morrison, 17. "They were telling everyone to get out of the hallways."

Authorities said Andrel Napper, 17, and Andre Wilkins, 18, were shot from behind in a fourth-floor hallway. One was shot in the back and the other in the buttocks, and both were listed in serious condition.

A .380-caliber handgun was found on the fifth floor, along with three shell casings, authorities said.

The school was swiftly locked down and students kept in their classrooms as police in riot gear swept the building on Amsterdam Avenue between 65th and 66th streets.

"They wanted us to be in the room, locked up until they had things under control," said senior Giorlys Alvarez, 17, who was locked in a classroom for more than an hour.

The school's metal detectors and X-ray machines were working yesterday, Police Commissioner Ray Kelly said.

Teacher Octavia Melian said the school had previous problems with students breaching security. She said students would pass through the metal detectors, then open side doors for their friends to smuggle guns and other items inside.

"It's been a constant concern with the faculty and the student body," she said.

The shooting was the first inside a New York City public school since September 1994, according to the teachers' union. The school district is the nation's largest, with more than a million pupils and 80,000 teachers.