The US embassy in Moscow has expressed concern over the detention of a Russian opposition activist in Kiev and his transfer to Moscow.
Leonid Razvozzhayev was apparently snatched off the street on Friday in the Ukrainian capital as he sought UN advice about political asylum.
He is accused by Russian investigators of plotting mass disorder in Russia.
Mr Razvozzhayev insists he was kidnapped and tortured but Russian officials say he handed himself in.
Ukraine's interior ministry said police had not got involved because it appeared that Mr Razvozzhayev had been detained not by criminals but by "secret services". There was no immediate comment from the Ukrainian foreign ministry.
A wider investigation is under way in Russia into opposition activists, sparked by allegations made against them in a recent TV documentary.
There are fears among human rights activists and opposition leaders that Russia's legal machinery is being used to repress political dissent.
- Thursday (18 October): Russian arrest warrant issued for Leonid Razvozzhayev
- Friday: Razvozzhayev disappears off the street in Kiev, Ukraine, while taking a break from talks on seeking political asylum
- Friday-Sunday: Razvozzhayev (by his own account) abducted by masked men and tortured for two days at an unknown location, before being forced to sign a confession
- Sunday: According to Russian investigators, Razvozzhayev hands himself in to them in Moscow
- Tuesday: Human rights activists are allowed to visit Razvozzhayev in prison in Moscow