"This is unacceptable ... I bear full responsibility for this," RIA Novosti quoted him as saying.
Wals noted that he deeply apologizes to reporters.
He also called their detention "unintentional."
According to the agency, a number of journalists reported police attacks in Minneapolis.
Among those attacked by the police were CBS, NBC, VICE, Minneapolis Star Tribune, Los Angeles Times.
A RIA Novosti correspondent also suffered, tear gas was sprayed in his face.
The Russian Foreign Ministry called the use of police against journalists unacceptable. They called on relevant international structures and human rights NGOs to "properly respond to this arbitrariness."