TACOMA, Washington (AP) – A procession of 2,000 cars followed the flag-draped caskets of four slain police officers to a memorial service yesterday as thousands of mourners, mostly members of law enforcement from across the country, lined the streets.

Lori Lightfoot, a police detective from Fresno, California, was among officers who travelled from as far away as New York, Chicago and Canada to remember Sgt. Mark Renninger and Officers Ronald Owens, Tina Griswold, and Greg Richards.

Lightfoot and Sheila Chandler, also a Fresno detective, said the deaths of the four Lakewood officers brought back memories of four Oakland police officers killed during a traffic stop and a shootout in March.

"It's just disbelief," Lightfoot said. "It's unbelievable that it could happen again."

The Lakewood officers were killed on November 29 before the start of their shift. Authorities say Maurice Clemmons singled them out and spared employees and other customers at the coffee shop in Parkland, a Tacoma suburb about 35 miles south of Seattle.

Clemmons was shot to death last week by a Seattle police officer after a two-day manhunt. Prosecutors said he received help from family and friends, and seven people have been arrested.

Several candlelight vigils have been held for the officers since the shooting, but yesterday's memorial service and procession is believed to be the largest in state history. Thousands of people lined the streets around the Tacoma Dome, and an estimated 20,000 people attended the service, which was delayed more than an hour because of the large law enforcement procession.

The officers' family and friends served as pallbearers, carrying the coffins inside.

A ceremonial honour guard opened the ceremony, and every officer in the arena stood at attention and saluted as the families of the four officers and members of the Lakeland Police Department were led to their seats.

"They were good people, and they were great cops," Lakewood Police Chief Brett Farrar said at the service. "And they will be truly missed."

In addition to eulogies from family, friends and public officials, mourners will watch a video tribute to the officers. Floral arrangements lined the stage, as well as two motorcycles, a drum set and a NASCAR race car.

Cpl. Jack Hundial, of Surrey, British Columbia, was one of 1,000 Royal Canadian Mounted Police in attendance. He said he and his colleagues wanted to show their support because "it could have been any of us."

"I think about their families," he said. "I don't think you ever find true closure for something this tragic."

Gov. Chris Gregoire, who will speak at the service, has directed that flags at all state facilities be lowered to half-staff Tuesday.

Additional space for the public to view telecasts of the service was made available at the University of Washington Tacoma, Pacific Lutheran University and the Christian Faith Center in Federal Way.