A patient in a psychiatric center near Massachusetts General Hospital stabbed his doctor this afternoon before being shot dead by an off-duty security guard, police said.

The physician is in stable condition and the patient died from the gunshot wound, Police Commissioner Edward F. Davis said at a press conference this afternoon as he described a chaotic scene.

The actions of the security guard, who does not work at the hospital, were hailed as "heroic" by Bonnie Michelman, head of security for Massachusetts General Hospital.

"We're happy he was there," Michelman said.

The violence occurred just after 2 p.m. on the fifth floor at 50 Staniford St., an office building where Massachusetts General Hospital leases space for its Bipolar Clinic and Research Program. The building is a few blocks from the main hospital building on Fruit Street, where an unrelated assault occurred last week.

This afternoon, police and witnesses described a crime scene crowded with medical personnel and patients that stretched from an interior office to a hallway.

"There was blood on the rug," said David Schoenfeld, a biostatistician who was working in an adjacent office on the fifth floor when the treatment session turned violent. "The man was tall, heavy-set, white, and looked about 35 to 40 years old. He was dressed casually, in jeans."

After at least one gunshot echoed on the fifth floor, two nurses from Schoenfeld's office went to treat the patient, who had apparently been shot in the head by the security guard, Schoenfeld said.

The patient was still breathing when paramedics arrived. "It felt like it took a long time before the ambulances arrived, but it was probably only a few minutes," Schoenfeld said.

At the press conference, Davis said that a male psychiatric patient attacked his female physician during a treatment session, stabbing her with a knife.

"During the course of the stabbing incident, an off-duty security officer who was armed interceded," Davis said. "He produced a weapon and ordered the suspect to drop the knife. When the suspect did not comply, he shot the suspect."

Police did not release the names of the doctor, patient, or the security guard. The doctor was treated at the hospital and is in stable condition. The patient was pronounced dead. The security guard is being questioned by police.

"They just put everything on lockdown and said no one can leave the floor. No one knew what was going on," said a 21-year-old ophthalmology technician, who worked on the sixth floor who declined to give his name. "No one knew what was going on."

"You didn't know what was going on, but you knew something happened," he said.

This was the second attack in less than a week inside a Massachusetts General Hospital facility. On Thursday afternoon, a 40-year-old homeless man allegedly assaulted a 27-year-old female employee inside a restroom at the main hospital building on Fruit Street. The suspect, David Flavell, a repeat sex offender, was ordered today to undergo a 20-day mental competency evaluation at Bridgewater State Hospital.

In 2003, a woman who worked with a prominent cardiologist at Massachusetts General Hospital fatally shot the doctor and then killed herself in his office near the hospital's main lobby, police said at the time. Authorities were never unable to determine a motive. Hospital officials said there were no complaints about the woman's work performance or behavior and no indication that the woman and the doctor had anything other than a professional relationship.