Russian opposition activist Konstantin Sinitsyn has been found dead of head injuries in the entranceway to his St. Petersburg apartment building.

St. Petersburg Legislative Assembly lawmaker Boris Vishnevsky posted the news on February 2, although the incident apparently happened on January 26.

Vishnevsky said Sinitsyn died of trauma to the head and that police had detained one suspect. Authorities say they believe the motive was robbery.

Sinitsyn, 53, was a regular participant in pro-democracy and anticorruption demonstrations in St. Petersburg.

In 2015, he supported a wave of protests conducted by long-distance truckers against rising road tariffs. He also figured prominently in St. Petersburg protests against a government decision to hand the landmark St. Isaac's Cathedral over to the Russian Orthodox Church.

With reporting by Fontanka.ru