As police officers from the city's Highland Park station filed into Oakland's Petersen Events Center yesterday to honor their slain comrades, Sgt. Craig Campbell swapped plainclothes for a uniform to take their place on the streets.

Normally an undercover narcotics detective, he and countless officers from other precincts and the bureau worked patrol shifts at Zone 5, attending the memorial only in spirit.

Behind the stacks of flowers, wreaths and cards that marked the station's entrance, they said they volunteered to work the extra shifts so Zone 5 officers -- who worked alongside Officers Eric G. Kelly, Stephen J. Mayhle and Paul J. Sciullo II -- could pay their respects in person.

"Somebody's got to keep the door open," Sgt. Campbell said, watching the service on television while listening to his police radio. "Everyone here wants to be there. But the nature of our job is the door always has to be open."

Officers worked long hours in different roles and locations than they usually would, some patrolling after years away from the streets. Burglary investigators, members of the auto squad, plainclothes detectives and supervisors from the city's five other stations milled about Zone 5 yesterday afternoon, some behind desks, others readying to hit the road.

"We came back and put on the uniform," said Sgt. Doug Epler, who also works undercover in narcotics. "Everyone is street police first."

Outside the station sat two fire trucks with ladders raised to form an archway in anticipation of a procession of hundreds of police vehicles. A traffic sign flashed the names of the slain officers in digital, orange letters, calling them "fallen heroes."

Most officers working yesterday agreed that keeping busy didn't ease their sense of loss. They said their thoughts would be with their colleagues, even as they responded to the night's various calls.

"Those of us who have been here, this is our way of paying tribute," said Sgt. William Vollberg, who normally works out of the Squirrel Hill station. He said officers have volunteered to work additional shifts throughout the week and will continue to do so, ensuring everyone has a chance to get to the week's services. "Everyone is trying to do their part."

"It's our obligation," Detective John McBurney said. "We're our brothers' keepers."

Police officers from other area departments stayed on the job yesterday, too, while their colleagues from around the country flocked to the memorial.

As the procession moved though Oakland, Port Authority police Officer Matt Geffel directed traffic, chatting with passing officers. He felt badly that he couldn't attend the memorial service, and struggled to put his feelings into words.

"As a father of young children, I can't describe how it feels," he said. "If we go, we go doing our jobs. ... It's the ones you leave behind that you worry for. Not for yourself."