FERGUSON, Mo. — The St. Louis County executive declared a state of emergency here on Monday as officials and activists sought to regain control of the volatile streets after plainclothes police officers shot and critically wounded an 18-year-old black man who they said was firing on them late the night before.

The police said the man, Tyrone Harris Jr., was among two groups of young people who exchanged gunfire near peaceful protests late Sunday on the first anniversary of the death of Michael Brown, an unarmed teenager who was killed by a white police officer in Ferguson. Prosecutors on Monday charged Mr. Harris, of the St. Louis suburb Northwoods, with 10 counts, including four of felony assault on a law enforcement officer.

The declaration of a state of emergency by the county executive, Steve Stenger, empowered the county police force and its top commander, Chief Jon Belmar, to oversee police operations in and around Ferguson, where police units from surrounding towns arrived on Monday to bolster efforts to maintain calm. Gunfire on the fringes of demonstrations commemorating Mr. Brown’s death — which set off looting, arson and confrontations with the police last year — unnerved residents and demonstrators over the weekend.