Ukraine's SBU security service said that 22-year-old Maria Koleda had been arrested in southern Ukraine "while carrying out a mission from the (Russian) secret services to destabilise the situation".
When arrested, Koleda was carrying written instructions on how to train sabotage groups, as well as a non-lethal pistol that had been adapted to fire live ammunition, the SBU said.
Local media reported that several people were shot during clashes on Monday outside government offices in the southern city of Mykolayiv between pro-Russians and supporters of the new authorities in Kiev.
It said Koleda had told her Russian "spymaster" that her fighters had an "unlimited supply" of home-made explosives.
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It said Koleda was sending a 13-person "sabotage group" to the eastern city of Donetsk, where pro-Kremlin protesters took over government offices at the weekend and declared independence.
Russian media reported that she had switched sides from a radical opposition group to supporting Kremlin policies.
Koleda can be seen posing in khaki clothes with an army-issue sniper's rifle on her VKontakte social networking page, her red hair in a pony tail.
The news website Slon.Ru said Koleda had been a member of radical groups Other Russia and the Avant Garde of Red Youth before switching to a pro-Kremlin group, Young Russia.
In 2008, Koleda took part in a raid by Other Russia activists on the Russian foreign ministry.
Other Russia activist Andrei Pesotsky told Fontanka.Ru website: "Koleda got involved in politics at 13 and frequently changed her views."
He described her as "inclined to militarism".
In her last post on VKontakte, written in the early hours today, Koleda wrote a message to pro-Russian protesters: "Don't despair... Everything is ahead."
The post included photographs from Kiev and Mykolayiv this month.