A murder investigation was underway Wednesday after an attempt to stop a stolen snowplow during a wild police chase through snowy city streets ended in the death of a police officer.

Sgt. Ryan Russell, an 11-year member of the force who was married with a two-year-old son, died of injuries sustained when he was struck by the plow.

"This is the worst of days for the Toronto Police Service," a sombre Chief Bill Blair said outside the hospital emergency entrance shortly after Russell, 35, succumbed.

"(He) put his life on the line and tragically has lost his life."

A man was arrested about seven kilometres from where Russell was hit.

Blair would only say the suspect received "certain injuries" during his arrest, but investigators confirmed police opened fire on him and he was hit by at least one bullet.

The man was taken to the same emergency room that attempted to save Russell's life.

The incident began at about 5 a.m. when someone stole a snowplow, which was equipped with a GPS tracking device, from a downtown parking lot near Dundas and Parliament streets.

Richard Eros, general manager of Tolias Landscaping and Plowing, said two operators were clearing a lot in the city's Regent Park neighbourhood when they got out of the truck.

Without warning, a man who appeared to be his 40s and without shoes jumped in and drove off in the Chevy 3500 pickup.

"They even were right beside the vehicle and they had a chance to grab the door and instantly it was gone before they even knew what had happened," Eros said.

The company alerted police to the stolen vehicle, which had a plow on the front and a salt dispenser on the back.

The plow, driven erratically, sped through snowy and slushy streets, hitting "numerous" vehicles on the way, police said.

About 40 minutes later at a midtown intersection, police tried to get the man to stop and Russell was struck.

Paramedics worked frantically on him at the scene before rushing him to hospital, where he was pronounced dead.

Officers finally caught up with the plow in the city's west end -- about two hours after the initial theft and some 10 kilometres from where it was stolen -- when it hit a city vehicle.

Heavily armed emergency officers made the arrest.

At a news conference later in the day, Blair refused to provide any specifics of the suspect, saying no charges had yet been laid.

"The murder of Sgt. Russell is being investigated by the Toronto homicide squad," Blair said.

Russell, whose father was also a police officer, had been promoted to sergeant six months ago after working with the city's guns and gangs task force.

Flags at city hall and civic buildings were lowered, and Russell's death prompted an outpouring of sympathy from citizens, on social media and other websites, politicians, and officers across the country.

Toronto Mayor Rob Ford called Russell a "shining example" of the men and women who put their lives on the line as officers.

"(They) earn the respect and appreciation of us all."

Eros said the tracking device used to keep tabs on the company's snow-clearing operations sends a signal via satellite to a computer at its office. That allowed him to notify police about the plow's whereabouts.

"Originally, it was just a vehicle stolen -- I thought we would have it recovered within the hour, maybe just abandoned and not a big deal whatsoever," Eros said.

"What transpired is actually a horrible tragedy."

Hours after the incident, police were still interviewing the two employees who were with the vehicle when it was stolen.

The provincial Special Investigations Unit was probing the incident.

"During the investigation of the driver, a number of shots were fired by police," the unit said in a statement.

"The driver sustained gunshot injuries and was taken by ambulance to St. Mike's Hospital in Toronto."

Mike McCormack, of the city's police union, called it a "tragic day," saying Russell had made the "ultimate sacrifice" doing his duty.

"We've lost a great officer, a great friend, a husband and a father," McCormack said.

"I can't imagine the grief that the family is going through right now."

Alok Mukherjee, chairman of the Toronto Police Services Board, said the city shared in the family's grief, and added such tragedies are rare but "affect all of us" when they occur.

Premier Dalton McGuinty called the tragedy a "grim reminder" that the dangers of policing cannot be taken for granted.

"Every once in a while we are reminded through tragic circumstances of the kind of work that we ask our police to do for us every single day," he said.

"And today's another painful example."

Public Safety Minister Vic Toews issued a statement Wednesday night extending condolences to Russell's family, friends and colleagues.

"His heartbreaking loss is a reminder to all Canadians of the unremitting commitment of our police, and the constant threats our officers face daily to keep our families and communities safe," Tiews said.

Funeral arrangements had not been made yet although officers from across North America were expected in the city to show their respects.

Leave a message of condolence for Sgt. Ryan Russell's family here.