Ukrainian activist group Femen reports that one of its leaders, Anna Hutsol, has been detained at a Russian airport.
The group said on its Facebook page on November 16 that Russia's Federal Security Service (FSB) was responsible for the detention.
A subsequent report by "Ukrayinska Pravda" said FSB agents and border guards took Hutsol's documents at St. Petersburg's Pulkovo Airport and took her away for questioning.
Femen promotes women's rights through high-profile stunts that are frequently carried out topless, attracting wide international attention.
The group has also displeased officials at home in Ukraine, in neighboring Belarus, and in other countries where they have staged protests, often considered by some to be disrespectful and in some cases, illegal.
Their targets are frequently perceived abusers of women's rights, but the group, which has grown exponentially in recent years with branches in a number of countries, also takes on other issues.
Established in 2008, Femen has recently sought to disrupt events at the London Olympics and the Euro 2012 soccer tournament in Poland.
One member fled Ukraine after causing a scandal by cutting down an Orthodox cross in a park.