The unidentified young woman was photographed during protests in Santiago, marking 43 years since the bloody military coup, which resulted in President Salvador Allende being overthrown, killed, and replaced with a military government led by Augusto Pinochet.
The protesters were honoring the victims of the Pinochet regime -- the dictator was accused of ordering the torture and killing of thousands of people during his 17 years in power. He died in 2006 without ever being tried on the allegations.
The march was organized by an association representing relatives of the disappeared.
The photograph was taken by Reuters photographer Carlos Vera Mancilla outside the General Cemetery in Santiago in the commune of Recoleta, where the protesters had gathered.
Police began making arrests and Mancilla told Chilean news website La Tercera that police apprehended two of the woman's friends when she "reacted and stood before the policeman with a defiant look."
He did not ask the girl her name.
The photo has been shared tens of thousands of times on social media.
Some of those who reacted to the photograph compared it to the image of the unknown man who stood in front of a tank during the Tiananmen Square pro-democracy protest in Beijing in 1989.
Facebook user Chrissy Gilliam commented on the Santiago photograph, saying: "The conviction in her eyes is stronger than his and the gear he's wearing."
Francis Ibori wrote: "This is one of the most powerful images I've ever seen in my life."
Seven police officers were injured and around 49 people were detained during confrontations between police and the protesters.
The Interior Ministry said there were a total of 140 altercations but that these were less intense than last year's protests.