The governor's executive order came just moments after Missouri Highway Patrol Capt. Ron Johnson said new security measures were under discussion but did not include bringing in National Guard troops.
But Nixon, who declared a state of emergency in Ferguson Friday, said the move was necessitated by "the violent criminal acts of an organized and growing number of individuals, many from outside the community and state."
Johnson, whose appointment last week to take over security from local police initially calmed tensions, said a small group of agitators determined to "provoke a response" were to blame for Sunday night's unrest.
Speaking at an early morning news conference, Johnson said police responded to a shooting among the protesters around 8:25 p.m., nearly four hours before a midnight curfew was set to begin for the third straight night. He said shots were fired and molotov cocktails thrown at officers as businesses, including a McDonald's were looted.
Meanwhile, thousands who tuned into a live stream of the protests online watched as police appeared to warn protesters to back off, then firing canisters of tear gas just seconds later, while children were still among the crowd.
Several journalists reported being briefly detained by police and cuffed using plastic restraints. One officer pointed a gun at Argus radio's Mustafa Hussein, in full view of the live stream feed, threatening to shoot him if "he didn't get the [expletive] out of here."
Johnson said three protesters were injured, and a spokesman of the Highway Patrol said seven or eight people were arrested.