Amarion Allen, 11-years-old stands in front of a police line shortly before shots were fired in a police-officer involved shooting in Ferguson, Missouri August 9, 2015. [Photo/Agencies]

FERGUSON, Mo. - Authorities declared a state of emergency in Ferguson, Missouri, on Monday to prevent a repeat of the violence that erupted during protests overnight to mark the police shooting of an unarmed black man one year ago that ignited a national firestorm on race relations.

The order was issued for the St. Louis suburb and surrounding areas amid tensions between residents and police after officers shot and critically wounded an 18-year-old man in an exchange of gunfire that marred what had been a day of peaceful demonstrations.

Prosecutors charged the man, Tyrone Harris, who was in critical condition in a hospital, with four criminal counts, including "assault on law enforcement" and shooting at a motor vehicle. His bond was set at $250,000.

St. Louis County Executive Steve Stenger said he made the state of emergency declaration because of "the potential for harm to persons and property". It marked another chapter in the turmoil that has gripped Ferguson since Michael Brown, an unarmed black teenager, was shot dead by white officer Darren Wilson a year ago.

In Ferguson, a few merchants said they were ready to protect their businesses with firearms, while store owners pleaded for calm.