Lawyers Mark and Patricia McCloskey drew guns on racial justice protesters marching through the grounds of their $1.15m mansion last month.

The couple said they armed themselves because they felt threatened.

But St Louis' top prosecutor said their actions had risked creating violence at an otherwise peaceful protest.

"It is illegal to wave weapons in a threatening manner at those participating in non-violent protest, and while we are fortunate this situation did not escalate into deadly force, this type of conduct is unacceptable in St Louis," said Kim Gardner, who is the city's first black circuit attorney.

"We must protect the right to peacefully protest, and any attempt to chill it through intimidation will not be tolerated," she added.

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