Henry Germain, 62, of Dublin was arrested at the club without incident on Monday and was taken to the county jail in Dublin, where he is being held on a no-bail arrest warrant, Nelson said.
The multiple victims in the case allege that Germain, who has worked at Castlewood for 16 years, molested them while acting as their tennis coach, Nelson said. The victims were children of club members.
Nelson said the alleged incidents occurred from 1995 to 1999 and came to light when some of the victims approached the club so that club management could take employment action against Germain.
Castlewood officials then contacted the Alameda County Sheriff's office, which conducted an expedited investigation.
Nelson said when sheriff's investigators contacted the alleged victims, they described multiple incidents of abuse by Germain when they were young.
Investigators believe Germain may have also victimized other children and are urging families to come forward if they believe their children were abused.
Nelson said victims can call an anonymous tip line at (510) 667-3622 or contact Sgt. Raymond Kelly at (510) 667-3657 or Deputy Rick Gonzales at (510) 667-4906.
On its Web site, the club identifies Germain as its "head tennis professional" who teaches players of all levels.
Germain is scheduled to be arraigned in Alameda County Superior Court on Wednesday.