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A live evening news programme on Russia’s main state TV channel was interrupted on Monday by a woman who walked into the studio protesting against the war in Ukraine.
A presenter was speaking when a woman appeared on camera behind her holding a sign with the headline “No war” scrawled in English across the top, with a message in Russian below calling on people not to believe Moscow's propaganda.
The news programme quickly cut away to another scene.
An independent human rights group that monitors political arrests identified the woman as 43-year-old Marina Ovsyannikova.
The group, OVD-Info, posted on its website that Ms Ovsyannikova, who has identified herself as a former senior editor at the Channel One station, was taken into police custody.
According to reports, her lawyers initially said they had been unable to contact her since her detention.
Ms Ovsyannikova spoke out against the war in a pre-recorded video on OVD-Info’s website.
"Unfortunately in recent years I worked on Channel One, making Kremlin propaganda and I am now very ashamed of this," she said.
"I'm ashamed that I allowed lies to be spoken from the TV screen. I'm ashamed I allowed Russian people to be zombified.
"We were silent in 2014 when this was all just beginning," she said, apparently referring to Moscow's takeover of Crimea and support for Ukraine's pro-Russian separatists.
"We didn't go to protests when the Kremlin poisoned (Alexei) Navalny. We just silently observed this anti-human regime. And now the whole world has turned away from us."
She urged Russians to go out and demonstrate, adding that "they can't throw us all in jail".
Speaking in a video address early on Tuesday, Ukraine’s President Volodymyr Zelenskyy praised Ms Ovsyannikova.
People in Russia have limited access to information from outside their country.
"I am grateful to those Russians who do not stop trying to convey the truth. To those who fight disinformation and tell the truth, real facts to their friends and loved ones," Mr Zelenskyy said. "And personally to the woman who entered the studio of Channel One with a poster against the war."
Kira Yarmysh, spokeswoman for jailed opposition leader Alexei Navalny, wrote on Twitter: "Wow, that girl is cool."
She posted a video of the incident, which quickly racked up more than 2.6 million views.
In a statement carried by state news agency TASS, Channel One said "an incident took place with an extraneous woman in shot. An internal check is being carried out".
TASS cited a law enforcement source as saying the woman has been detained and could be charged under legislation banning public acts that aim to "discredit the use of Russia's armed forces".
