British newspaper The Daily Telegraph earlier reported that an official at the Serbsky Institute for Social and Forensic Psychiatry in central Moscow said Bukovsky, who now lives in London and spent long periods in the 1960s and 70s in Soviet psychiatric wards for criticizing authorities, was "undoubtedly psychopathic".
"The initiative group nominating Vladimir Bukovsky as a [March 2008] presidential candidate is demanding that the Serbsky Institute immediately refute the statement made by its representative, published in The Daily Telegraph," the group said threatening to initiate a libel suit.
Bukovsky, 64, who spent 12 years in prisons, labor camps and psychiatric units, was one of the first to expose psychiatric imprisonment as a tool to break dissidents in the Soviet Union.
"After his arrest he wrote hundreds of letters of complaint," The Daily Telegraph quoted the official as saying. "Not every person would do that. It was another symptom of his condition."
The initiative group said the statement could have been thought up by the Kremlin as a pretext to deny him registration citing mental illness. The Constitutional Court is currently considering whether to permit candidates with dual citizenship to run in elections.
Deported to Britain in 1976, Bukovsky received his Russian citizenship back in 1992 from former President Boris Yeltsin.
The newspaper also raised concerns that "punitive psychiatry" is on the verge of a making a comeback in Russia, which has a negative democratic record under President Vladimir Putin.
The periodical recalled last month's detention of Larisa Arap, 49, a Murmansk human rights advocate, by mental health authorities for what the newspaper presumed was her exposure of appalling standards of hospital child care and treatment.