St. John’s University was locked down this afternoon after a man carrying a rifle and donned in a rubber George Bush mask was arrested at its Queens campus, police and university authorities said.

Before he was arrested, the man, identified by police as Omesh Hiraman, 22, of Queens, was seen striding quickly across campus in the mask streaked in red with the mouth cut out.

A police cadet, Christopher Benson, 22, who is also a student at St. John’s, followed Mr. Hiraman after noticing he was carrying a rifle wrapped in a black plastic bag. Mr. Benson tackled him as a security guard approached Mr. Hiraman and tried to wrest away the gun.

“When the struggle happened I just jumped right in,” Mr. Benson said, who said he had often seen Mr. Hiraman, a freshman at St. John’s, around campus.

The security guard seized the gun — a 50-caliber rifle — after a brief struggle. No shots were fired and no one was injured. Mr. Hiraman was arrested and charges against him are pending.

Police officials said they believe the gun, a modern rifle similar to Revolutionary War style shotguns, had been purchased earlier in the day, and described it as a “very, very powerful weapon.” The gun had been charged with gunpowder, but Mr. Hiraman wasn’t carrying other ammunition, police said.

Police said they were searching Mr. Hiraman’s home in the Elmhurst section of Queens this evening for additional ammunition. Police have not yet determined whether the gun was fully loaded.

Police officers flooded the campus after the arrest, university officials said, and searched all of the university’s buildings and facilities. Students and staff were told to stay inside the classrooms and buildings for several hours. Evening classes were canceled.

“We’re just told to stay where we are. Everybody seems to be listening to what they’re saying,” an employee at St. John’s, Mary Scholfield, said speaking on a phone from inside one of the locked-down buildings. “I do feel secure — there’s all these policeman and swat team members. You do feel relatively safe.”

The university sent a notification about the incident to students via cell phone voice and text messages using a new emergency system introduced two weeks ago in response to the recent shootings at Virginia Tech. Mr. Benson said he received the emergency text on his phone as he was struggling to pin down the suspect and take his gun.

The incident comes on the heels of a conference called by Police Commissioner Raymond Kelly to address safety at New York City campuses following school shootings elsewhere in the country. St. John’s security officers attended.