A dissenting employee entered the studio Monday during Russia's most-watched evening news broadcast, holding up a poster saying "No War" and condemning Moscow's military action in Ukraine.
The incident was a highly unusual breach of security at the tightly controled state broadcaster Channel One. Its flagship 9:00pm news show called "Time" has run since the Soviet era and is watched by millions around the country, particularly by older Russians.
OVD-Info, which monitors detentions at opposition protests, identified the woman as Marina Ovsyannikova, saying she works at Channel One as an editor and was now at a police station.
As the news anchor Yekaterina Andreyeva launched into an item about relations with Belarus, Ovsyannikova, who wore a dark formal suit, burst into view, holding up a hand-written poster saying "No War" in English.
Below, the poster said in Russian: "Stop the war. Don't believe the propaganda. Here they are lying to you." It is signed in English: "Russians against the war."
The protester managed to say a few phrases in Russian, including "Stop the war!", while Andreyeva, who has presented the news since 1998, tried to drown her out by speaking louder.
The channel then switched hastily to footage of a hospital.
In a statement carried by state news agency TASS, Channel One said that "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."
'Zombified Russian people'
OVD-Info posted a video where Ovsyannikova says her father is Ukrainian and her mother is Russian and she does not see the countries as enemies.
"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," she added.
"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."
Russia has blocked or limited popular social media platforms Facebook, Twitter and Instagram, all of which were widely used to make political statements.
A video clip of the incident spread quickly on social media, with many users paying tribute to the woman's "extraordinary courage" against a backdrop of a heavy crackdown on opposition.
Since the start of the intervention in Ukraine on Feb. 24, thousands of protesters have been arrested in Russia.
Leonid Volkov, who is close to Navalny, the opposition leader who has been imprisoned since last year after surviving a poisoning, tweeted that his movement "is ready to pay any fine" imposed on Ovsyannikova.