Oakland police officers' association have issued a statement criticising the city's mayor and administration on the eve of an Occupy strike which has pledged to "shut down" the downtown area.

In an open letter to the citizens of Oakland the association, which represents all 645 Oakland police officers, accused Mayor Quan of "staffing" the strike on Wednesday and said the administration was sending out "mixed messages".

Police in Oakland have faced heavy criticism after they used tear gas and 'less lethal' weapons to clear the Occupy Oakland protest last week, an operation which left ex-marine Scott Olsen seriously injured. Wednesday's strike is expected to see thousands of protesters march on the Port of Oakland in a bid to shut down trade in the city.

"We, too, are the 99% fighting for better working conditions, fair treatment and the ability to provide a living for our children and families," the OPOA statement said – one of the first times a police force has officially adopted the '99%' language of Occupy protesters.

"We are severely understaffed with many City beats remaining unprotected by police during the day and evening hours."

The statement said officers had been left confused by decisions taken by Mayor Quan.

"On Tuesday, October 25th, we were ordered by Mayor Quan to clear out the encampments at Frank Ogawa Plaza and to keep protesters out of the Plaza. We performed the job that the Mayor's Administration asked us to do, being fully aware that past protests in Oakland have resulted in rioting, violence and destruction of property.

"Then, on Wednesday, October 26th, the Mayor allowed protesters back in – to camp out at the very place they were evacuated from the day before."

Sergeant Barry Donelan, active police officer and vice-president of the OPOA, said while police would "absoutely not" be joining in any strike action – it is illegal for police to strike in California – the statement had been put out to illustrate a "leadership vacuum" in Oakland.

Public sector employees in Oakland have been told by the city administration that they have the right to strike on Wednesday – but the OPOA said this was a contradiction given all 645 police officers had been told they had to work.

"Aren't the mayor and her administration part of the establishment they are paying city employees to protest? Is it the City's intention to have City employees on both sides of a skirmish line?"

Donelan said while police support the right of free speech and the right to protest, officers "want clear speech".

"The city's gonna field two teams [on Wednesday], the blue team and on the strikers' team all the city employees."

Wednesday's strike, organised by Occupy Oakland, aims to "shut down" the city, culminating with a march to the Port of Oakland to prevent the transit of cargo. Workers, university students and school pupils are being urged to rally close to the Occupy camp, outside city hall, during the day before marching to the port at 5pm.