OAKLAND, Calif.—A former California police chief fired from his post last year has sued the city of Oakland and its mayor, saying he was unlawfully terminated in retaliation for criticizing the federal court-appointed monitor overseeing the department.

LeRonne Armstrong filed his lawsuit in Alameda County Superior Court on Feb. 5. He seeks reinstatement as police chief, the post Mayor Sheng Thao fired him from in February 2023 after a probe ordered by the oversight monitor found he mishandled two misconduct cases.

Oakland has been without a permanent police chief since, even as violent crime, robbery, and vehicle theft climbed in the city of 400,000 across the bay from San Francisco. On Feb. 6, Gov. Gavin Newsom announced he will deploy 120 California Highway Patrol officers to Oakland to assist with targeted crackdowns on criminal activity, including vehicle and retail theft.

Preliminary data shows that crime rose in Oakland last year, despite falling in other California urban centers, Mr. Newsom’s office said. Last month, In-N-Out Burger announced it would close its first location in its 75-year history due to car break-ins, property damage, theft, and robberies at its only restaurant in Oakland.

Oakland’s police department has been under federal oversight since 2003 after a rookie officer came forward to report abuse of power by a group of officers known as the Oakland “Riders.” The case resulted in the department being required to enact more than four dozen reform measures and report its progress to an outside monitor and a federal judge.

The mayor said in firing Mr. Armstrong last February that she had lost confidence in the police chief after he and the department failed to properly investigate and discipline a sergeant who was involved in a hit-and-run with his patrol car and who, in a separate incident, fired his service weapon inside an elevator at police headquarters.

In his complaint, Mr. Armstrong says the department had made great strides and was on track to regain its independence when the federal monitor said there were problems with police leadership and ordered the outside investigation into the sergeant. Mr. Armstrong says the monitor and his team “transformed routine instances of lower-level misconduct into a complete indictment” of the department and chief.

Mr. Armstrong said in his complaint that the mayor, who was newly elected at the time, was intimidated by the oversight monitor and buckled to pressure.

Ms. Thao’s office on Wednesday referred requests for comment to the city attorney’s office, which said in a statement that it had not been served with the complaint.