The St. Louis County Police Department on Friday took control of security surrounding protests in Ferguson, Mo., which have persisted since the death of Michael Brown, 18, at the hands of a police officer on Aug. 9.

Chief Thomas Jackson of the Ferguson Police Department asked the county to step in, citing a “lack of manpower and resources” at the disposal of the relatively small Ferguson police force, said Brian Schellman, a spokesman for the county police.

In a statement, the county force said it intended to “provide both our citizens and the media with the safest environment possible so that they may freely exercise their First Amendment right, while also protecting the rights and safety of all residents and businesses in the city of Ferguson.”

The shift means that the responsibility for making arrests, booking and releasing prisoners and filing charges will now be handled by the county force, a significantly larger and more sophisticated police department that is the main law-enforcement agency for much of the county. Some municipalities in St. Louis County, like Ferguson, maintain their own, smaller forces, but depend on the county police for handling more serious criminal activity. The St. Louis County police are in charge of the investigation into Mr. Brown’s death.