Andrei Yurov, who travelled to Minsk this week to assess the human rights situation in Belarus, was detained on March 16.
After he was released, Yurov told RFE/RL that he was officially informed that his name had been included in a list of individuals banned from entering Belarus.
Yurov added that he was officially asked to leave the country in 24 hours. He described as absurd a situation in which individuals are added to a blacklist not by a court but by some unknown institutions.
He said he thought his visit was legal since "there are no borders between Russia and Belarus," a reference to the two countries' visa-free regime that means there is no passport control for passengers arriving in Belarus from Russia.
He said he is awaiting a reaction to his case from rights bodies such as the Council of Europe and the Organization for Security and Cooperation in Europe.
Vadim Gusev, an official at the Russian Embassy in Minsk, told RFE/RL that the Embassy has sent an official note to the Belarusian Foreign Ministry requesting an explanation for Yurov's detention. Gusev added that the Russian Embassy will do its best to protect Yurov's rights.
Belarusian Foreign Ministry spokesman Andrey Savinykh told Interfax that he knew nothing about a list of Russian citizens barred from entering Belarus.
He said Minsk informed the Russian Embassy about Yurov's detention and directed further questions to law enforcement.
Read more in Belarusian here