At least nine protesters were arrested from Monday night into Tuesday morning for attempting to block a roadway, according to St. Louis County police.

Social media users decried the state of emergency, questioning why three armed white men - purported to be part of the armed conservative group Oath Keepers - were not arrested.

Last year, Darren Wilson - a white police officer - shot and killed 18-year-old Michael Brown, prompting a national outcry over race relations in the US. Monday's protests mark the fourth night of ongoing demonstrations for the one year anniversary of the his death.

State of emergency

On Monday, St. Louis County executive Steve Stenger announced that police would be in charge of "emergency management" in Ferguson after 18-year old Tyrone Harris allegedly fired shots at undercover police.

"In light of last night's violence and unrest in the city of Ferguson, and the potential for harm to persons and property, I am exercising my authority as county executive to issue a state of emergency, effective immediately," Stenger said in a statement.

Meanwhile, Harris' father said that his son was unarmed and questioned the legitimacy of the police's narrative.

Harris remains in critical condition at a hospital after sustaining fire from police.

'Black Lives Matter'

Michael Brown's death and the decision by a grand jury to not indict Wilson have led to a wave of mass protests across the US in response to white law enforcement officers killing unarmed black Americans.

The movement has gained notoriety for decrying the deaths of Eric Garner in New York and Freddie Gray in Baltimore, among others.

ls/kms (AFP, AP, Reuters)