Members of the Oath Keepers walk with their personal weapons on the street during protests in Ferguson, in the early hours of 11 AugustWearing bullet-proof vests, branded hats and camouflage gear, they walked among protesters after police appeared to have left the immediate area.
Kayla Reed, a prominent civil rights activist involved with the Black Lives Matter movement - who had been arrested and released earlier on Monday - asked on Twitter: “Why are there men with guns and the police are doing nothing?”
Fellow activist Johnetta Elzie, who had also been arrested, asked if black people would have been able to carry high-powered guns around St Louis so freely.
St Louis County Police Chief Jon Belmar called the Oath Keepers' presence "both unnecessary and inflammatory," according toNBC News.
Who are the Oath Keepers?The Oath Keepers, whose members were seen claiming to guard homes and businesses from rooftops during protests in Ferguson last year, have been accused of vigilantism by opponents but claim they are exercising Americans’ democratic rights.
Formed by a former US Army paratrooper, members pledge to fulfil the oath taken by the country’s military and police to “defend the Constitution against all enemies, foreign and domestic”, even if that means defying other laws and orders.
A former police officer gave a speech to the group's St Louis chapter last year where he was filmed saying: "“I'm also a killer. I've killed a lot, and if I need to I'll kill a whole bunch more. If you don't want to get killed, don't show up in front of me.
“I’m into diversity – I kill everybody”.
Alarmed protesters asked the Oath Keepers to leave on Monday, with some filming conversations where the men claimed they were on the demonstrators’ side and defending civil rights.
"If you're armed, why can't the protesters be armed?" one demonstrator asked.
The organisation has not released an official statement on its presence in Ferguson or intentions in the area.
Is it legal to carry guns in Ferguson?Under normal circumstances, yes, although it was unclear whether those laws could be affected by the current state of emergency.
Missouri is one of the US states with what is known as an "open carry" law, meaning that anyone legally possessing a gun can display it openly in public.
Proponents of the rule argue that criminals normally conceal such weapons and that people should not be afraid of law-abiding gun owners displaying them.
The practice is strongly opposed by advocates of increased gun controls in the US, who argue that the practice is intimidating, menacing and unnecessary.
Were Monday's protests violent?The vast majority of action on Monday was peaceful, although a small minority of protesters were seen hurling missiles including frozen bottles of water at police, who responded with pepper spray.
A spokesperson for the St Louis County Police Department wrote on Twitter: “Unruly crowd is throwing frozen water bottles at officers. Those who choose to act violently will be arrested.
“Safety, our top priority, is now compromised. This is no longer a peaceful protest. Participants are now unlawfully assembled.”
About 200 demonstrators, some waving flags, beating drums and shouting slogans had marched along a street that was a flashpoint of protests that erupted last year after white police officer Darren Wilson shot dead an unarmed 18-year-old black man.
Officers carrying shields rushed into a crowd of protesters around midnight local time, many of whom started screaming and running from the area as they were ordered off the streets.
A state of emergency was declared by authorities yesterday in Ferguson after several police officers shot a man in what they said was an exchange of gunfire on Sunday night. He is in a critical condition.
Scenes on Monday were calmer, with groups of protesters filmed singing and dancing in the street.
“What do we want? Justice. When do we want it? Now,” they sang, before chanting Mike Brown’s name.Officers and protesters face off along West Florissant Avenue on Monday.
How many people have been arrested?More than a dozen people were arrested in West Florissant overnight, on top of almost 120 arrests the previous day.
Princeton University professor and activist Cornel West and members of the local clergy were among those detained after walking through a temporary barrier to sit outside the Thomas Eagleton Federal Courthouse.
Richard Callahan, the attorney for the eastern district of Missouri, said 57 protesters were arrested for “obstructing normal use of the entrances” to the building in an “otherwise” peaceful and non-violent event. Many have since been released.
Mr Brown's father, Michael Brown Sr. wrote on Facebook that peaceful weekend protests were “meaningful, inspiring and successful.”
“With your support, we properly honoured your friend and my son's memory,” he wrote.