St. John’s University was locked down yesterday afternoon after a masked man carrying a rifle 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 across the campus wearing a rubber President Bush mask streaked in red with the mouth cut out.

A police cadet who is also a student at St. John’s, Christopher Benson, 22, followed Mr. Hiraman after noticing he was also 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, who said he has often seen Mr. Hiraman around campus, said.

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, a freshman at St. John’s, was arrested and charges against him were pending as of yesterday evening.

Police officials said they believed the gun, a modern rifle similar to Revolutionary War-style shotguns, was purchased earlier in the day, and they described it as a “very, very powerful weapon.” The gun was charged with gunpowder, but Mr. Hiraman did not have other ammunition with him, police said.

Police said they were searching Mr. Hiraman’s home in the Elmhurst section of Queens yesterday evening for additional ammunition. As of last night, police had 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 university staff were told to stay inside the classrooms and buildings for several hours, and 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 yesterday evening, 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 out 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 shootings last spring at Virginia Tech. Mr. Benson said he received the emergency text message 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 the police commissioner, Raymond Kelly, to address safety at city campuses following school shootings elsewhere in the country, which St. John’s security officers attended.