The Council of Europe has approved two reports strongly condemning the human rights situation in Belarus.
President Lukashenko is singled out for criticism
The council accused several Belarus officials of direct involvement in the disappearances of four men and called on member states to consider sanctions.
The men who vanished were all high-profile figures, including opponents of President Alexander Lukashenko.
The reports came shortly after a leading opposition figure was detained in Belarus on document theft charges.
Both reports were passed by a vast majority in the council's parliamentary assembly in Strasbourg.
But the Russian delegation voted against, warning that the council was adopting the wrong tactic.
The reports accuse Belarus authorities of blocking a proper investigation into the fate of four men who disappeared in the capital, Minsk, in 1999 and 2000: two politicians, a businessman and a cameraman.
They are presumed to have been abducted.
The reports name several senior government officials suspected of involvement and even single out President Lukashenko himself.
Following the assembly's debate, the author of the reports, Christos Pourgourides, said he had "no doubt" that the president himself gave the order for a cover-up.
Belarus officials deny the charges, saying they are doing everything possible to find the disappeared.
But in Strasbourg, the wife of one of the men called Wednesday's vote "a victory for democracy".
The BBC's Sarah Rainsford in Moscow says Russia, which supports Minsk economically, remains the dissenting voice in the debate.
Russian delegates believe isolating President Lukashenko would only provoke him into an even tougher response, our correspondent says.
Meanwhile, a politician who once ran against Mr Lukashenko in the presidential elections was held on charges of stealing official documents and unlawful handling of firearms, officials announced.
Opposition politicians criticised the arrest of Mikhail Marinich - a former minister, ambassador and mayor of Minsk.
"This arrest marks yet another wave of open political repression against civil society and democratic forces in Belarus," three opposition deputies said in a statement, quoted by AFP news agency.
Valery Frolov, Vladimir Parfenovich and Sergei Skrebets
argued that "all of Belarus society is in danger."