Russian police on Thursday detained four people over an arson attack linked to an upcoming controversial film about tsar Nicholas II and his affair with a ballerina.
Hardline conservatives have derided filmmaker Alexei Uchitel's movie Matilda and it has sparked protests.
A car was set on fire last week near the Moscow office of a lawyer representing Uchitel, whose own office in Saint Petersburg has also been targeted.
The interior ministry said on its website that four people were held for questioning over the Moscow arson.
Spokeswoman Irina Volk later told TASS agency that Alexander Kalinin, the head of the radical Orthodox group Christian State "has been detained as a suspect" along with three other people.
Kalinin last week admitted to AFP in an interview that his activists had made threats against Russian cinema chains to get them to drop the film.
Police said flyers found at the scene of the attack with the slogan "Burn for Matilda" were seized from Kalinin's flat, according to the Fontanka newspaper.
Matilda, due to be released on October 26, focuses on Nicholas II's relationship with ballerina Mathilde Kschessinska, which took place before his coronation and marriage.
Orthodox groups who revere the tsar, along with pro-Kremlin MP Natalia Poklonskaya, have criticised the film but the culture ministry nevertheless approved it for release.
A closed screening in Saint Petersburg on Wednesday went ahead amid tight security and following a police search of the cinema.
Culture minister Vladimir Medinsky had urged authorities to protect the film industry and thanked the police for intervening and "rapidly reacting to violations" on Thursday.
"I'm sure the situation will calm down," Medinsky was quoted as saying by Russian news agencies after the suspects' detentions.