Hamroev is said to have been in the southern part of the country in an effort to collect information on alleged abuses against Muslims, who make up a majority of Kyrgyzstan's population.
He is a member of the Moscow-based Memorial Human Rights Center, a leading rights-advocacy group that has been critical of government abuses throughout the former Soviet Union.
A Memorial representative said a local rights activist, Izzatilla Rahmatillaev, was detained along with Hamroev.
Vitaly Ponomarev, the director of Memorial's Central Asian program, told RFE/RL that Hamroev's mobile phone and camera were confiscated by police in Osh and he was put on a flight back to Moscow.
Rahmatillaev was later released.
Islamic extremists are frequently blamed for antistate activities in all five of Central Asia's post-Soviet republics, none of which receive high marks from the U.S. government or international NGOs for their rights records.