Sound the alarm - Oakland's Police Department is shrinking so fast that it doesn't have enough officers to cover some patrols and many of its investigative units have been stripped to the bone.

Everyone knows about the 80 officers the city laid off in July to save money. But since then, 21 more have retired, 12 have decamped for other police departments, five have simply quit and one has been fired - dropping the total number of officers to 670.

Meanwhile, 30 more officers are undergoing background checks by other departments seeking to hire them. And another 40 will be eligible to retire by year's end.

Even that doesn't tell the whole story.

Another 77 cops - or more than 10 percent of the entire force - are on the shelf because of injuries. That's about double the usual rate. Twenty will be going back to work in the next two weeks, but only for "light duty."

And thanks to a provision in a parcel tax that city voters passed in 2004, 63 cops have to be assigned as community problem-solving officers who ferret out trouble spots and crime trends in designated districts. That means they can't be assigned to investigations or to work in other neighborhoods.

Put it all together, and you have investigative units such as the burglary and robbery details being raided to fill patrol beats.

There are now just five cops investigating everything from auto thefts to burglaries to identity theft.

But even so, street coverage is becoming a challenge. On an average day, six of the city's 33 patrol car beats go uncovered for lack of officers.

Chief Anthony Batts - who estimated the city needs at least 925 cops to get the job done - is trying to make up for the loss by partnering up with federal, state and county law enforcement units.

"We're going to keep trying," he said. "We are not going to give up."

The fight isn't getting any easier.

After voters rejected a measure last month that would have helped hire back cops, City Administrator Dan Lindheim said Oakland could afford only 637 officers next year.

Considering the exodus under way, the city is likely to hit the 637 level - or even go lower - without the political embarrassment of more layoffs.

Mayor-elect Jean Quan has said she wants to hire more cops, but can do so only if officers agree to kick in 9 percent of their pay to help cover their pensions.

"The trouble is, the cops don't trust City Hall," said police union head Dom Arotzarena.

"There's no guarantee they won't just take the money and spend it elsewhere."