The head of Russia's Parnas opposition party, Mikhail Kasyanov, says a leading member of his party has been kidnapped by three unknown men in Moscow.

Kasyanov wrote on Facebook on April 26 that the kidnapped party official, Mikhail Konev, was the head of the Parnas youth wing.

According to Kasyanov, Konev was forced into a car as he was leaving a military commission building in Moscow, where he had been summoned on April 26.

Kasyanov also posted a photograph of the kidnappers' car on Facebook. The license plate of the car was legible.

Kasyanov said his party is demanding an immediate investigation into Konev's disappearance and has called on authorities to secure the young man's immediate release.

The Parnas party, led by Kasyanov, a former Russian prime minister, openly positions itself as a Kremlin opponent.

The former co-chairman of Parnas, Russia's former Deputy Prime Minister Boris Nemtsov, was shot dead near the Kremlin on February 27, 2015.