A 36-year-old man was being held today in connection with the slaying of James J. Riordan, the city police department's No. 2 man who was fatally shot as he tried to break up a disturbance in a downtown restaurant.

Informed of the shooting Saturday night, Mayor Jane Byrne rushed to Riordan's side at the hospital. As Riordan, still conscious, was being taken to surgery, the mayor told him, "'I love you, Jim,'" a spokesman said.

Riordan, 57, was pronounced dead after 90 minutes of emergency surgery. Doctors said three bullet wounds -- two in the neck and one in the chest -- had caused "irraparable damage" to major arteries.

The suspect, identified as Leon Washington, was taken into custody and charged with murder.

Police said Riordan was shot when he tried to break up a disturbance involving the suspect and a woman customer at a restaurant bar in the downtown Marina City condominium complex. The man was apprehended moments after the shooting, and a 9 mm pistol was recovered.

Police Superintendent Richard Brzeczek, who was appointed to head the department after Riordan turned down the post in 1979, took personal charge of the investigation.

A police spokesman said Riordan had stopped at the bar in the Marina City condominium complex with friends on his way home after working at police headquarters on contingency plans in case of a shutdown of Chicago's mass transit system.

While he was in the bar, a man approached a woman customer and began shouting at her, witnesses said.

Riordan and another patron tried to calm the man, witnesses said, and ushered him out of the restaurant into a hallway, where Riordan was shot.

The suspect tried to flee, police said, but was apprehended with moments by police officers in the building.

Riordan, a 34-year police veteran, was considered one of the department's most respected top officials. He was married and the father of four, including a son who is a police officer. During the 1968 Democratic National Convention, he was field commander of the city's police. Officers and patrolmen and Riordan was admired for his combination of administrative skills and experience as a street cop.

"He understood the street-level police officer," said Lt. John Gorman, who had worked with Riordan for almost 20 years. "He reacted as a policeman as well as a top administrator. People always gave that little extra for him. Even if he was chewing you out, he would never strip you of your dignity."