Civil liberties advocates say the city could save some of the millions of dollars paid out in lawsuits over police misconduct by strengthening the independent agency charged with investigating the police department.

The city has spent more than $300 million to resolve lawsuits brought by civilians for police misconduct since 2000, according to numbers compiled by the New York Civil Liberties Union from a report by the city comptroller last month.

“Taxpayers pay that money for unauthorized, inappropriate abuse of police power,” the principal author of a 63-page NYCLU report summarizing annual statistics released by the Civilian Complaint Review Board, Robert Perry, said. “By any measure the CCRB is failing.”

The CCRB conducts administrative, noncriminal investigations into allegations of misconduct.

The report argues that the CCRB has been lax in its investigations of police, pointing to a steady increase in complaints against police in recent years coupled with a decrease in cases that are fully investigated by the agency’s investigators. The report highlights old problems without noting recent improvements and “improperly applies statistics in order to unfairly malign the CCRB,” a statement by the CCRB said.

A spokesman for the police department, Deputy Commissioner Paul Browne, said the report unjustly attacks the CCRB and the police department for an increase in complaints after having criticized the department in years past — when complaints were down — for not doing enough outreach.

“Up or down, the NYCLU disingenuously accuses both the CCRB and NYPD of failure. Damned if we do. Damned if we don’t,” Mr. Browne said, who has said the city’s 311 hotline has made filing complaints easier, leading to the increase in complaints.

“Nothing will appease the NYCLU unless we fire every officer and go back to lawlessness,” he said.