The Moscow department of regional security has warned controversial LGBT rights activist Nikolai Alexeyev that participants in a rally planned for October 21 would be subject to arrest under the ban on "promoting non-traditional sexual relations to minors" in a letter dated October 9 and posted on Alexeyev's Facebook account on Friday.

The letter warned that the planned rally's location — in a square near the Church of Christ the Savior — "could be evaluated as an offense to the feelings of religious believers and provoke unlawful actions from people who do not share the views of its participants."

"Throughout the entire history of Christianity, the Orthodox church has recognized the traditional family, while homosexual relations have been interpreted by Christianity as a sin," the letter, signed by the deputy head of the department, V.V. Oleynik, on official letterhead, said.

"I warn you that the organizer, as well as other participants of the public event, could be held responsible in the appropriate order if the event is held with the stated goal," the letter closed.

Alexeyev had planned the rally to commemorate the third anniversary of his successful lawsuit at the European Court of Human Rights in Strasbourg challenging Russia's ban on Moscow's LGBT pride march.

"This is one of the strongest denials of my right to freedom of assembly I ever received... Shocking!" Alexeyev wrote on Facebook, vowing to challenge the letter in the European Court. "This will go to courts up to Strasbourg and also to the Committee of Ministers of the Council of Europe which monitors implementation of Alekseyev v. Russia verdict."

Alexeyev is among the most active and controversial LGBT activists in Russia.