“Safety, our top priority, is now compromised,” a spokesperson for the St Louis County Police Department wrote on Twitter.
“This is no longer a peaceful protest. Participants are now unlawfully assembled.”
In a Snapchat video posted online, Ms Kozhevsky said she wanted to protect the officers.
She said: “Look guys, discrimination is a thing and I get it but we need to stay with the people who protect us and that’s what I’m doing. I want to protect my police.”
The teenager, who studies nursing at St Louis University, told The Blazethat she believes “not all cops are bad” and that violent protests were not the way forward.
Ms Kozhevsky said officers questioned her after she moved in front of them but let her stay after she said she was aware of the possible danger.
“I was prepared for the worst,” she added.
“It was scary. People were looking at me and making gestures. It was unnerving, to say the least. But sometimes you just have to stand your ground.”
A photo of her went viral on Twitter, provoking strong responses from people following the events in Ferguson.
Ferguson Anniversary Protests
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Some contrasted her treatment to that of a black girl who was photographed being arrested. Online commentators claimed she was 12 but St Louis police later confirmed she was 18 after viewing her identification card.
“Lexi's allowed to occupy the street because of whiteness,” one person claimed.
“Other protesters have been forcibly removed from such spaces in Ferguson.”
Many people were critical of Ms Kozhevsky, saying her actions were unnecessary to protect “officers in full riot gear”.
But others praised her actions and told her to “stay strong” in the face of online abuse.
“I'm in awe at her courage,” one supporter wrote on Twitter, while others urged critics to “cut her some slack” and focus on bigger issues.
At least 23 people were arrested during Monday night’s protests, which saw officers in riot gear force protesters off the streets with shields and use pepper spray.
A state of emergency was declared by authorities 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.
The latest unrest comes a year after Mr Brown, an unarmed black teenager, was shot dead in the St Louis suburb by a white officer, sparking angry protests that ignited anger over police treatment of ethnic minorities throughout the US.
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