The important word is "public." There is little or no expectation of privacy in a public park.
The general advice above does not constitute an attorney-client relationship: you haven't hired me or given me confidential information by posting on this public forum, and my answer on this public forum does not constitute attorney-client advice.
Are you sure it is secret? Many times the state will use such video surveillance, but will be required to post signs alerting people of the cameras. Stop light cameras, for example. Contact an attorney in your jurisdiction to obtain information regarding state and municipal laws regarding the issue.
Please feel free to call Attorney Peter Blair for a FREE consultation. (619) 525-7005. Law Office of Vikas Bajaj, APC, specializing in Southern California Criminal Defense.