The incident happened at the Riverfront Music Festival in the Motor City, when "The Boy Is Mine" singer could tell from the stage that "things were heating up a bit," she told ABC News.
Video posted on social media showed two fans locked arm and arm in a heated argument, appearing to strike each other as others around them tried to break up the brawl.
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That's when Monica decided to take matters into her own hands, video shows.
"Wait a minute, wait a minute. Ah, ah, ah. No, don't you hit her like that," the 42-year-old singer can be heard saying before walking off the stage and heading into the audience.
"When I looked up from the mic, I saw a man strike what appeared to be an older woman," Monica told ABC News. "That, in itself, triggered me a bit, but it also was very scary because the last thing that I want is for someone to be harmed. When I jumped from the stage, it was simply to bring it all to a complete stop and make sure both people were OK, seperated and got home safely."
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Monica was later helped back up on stage as the crowd erupted in cheers.
She apologized to the crowd, saying the incident "triggered her."