Russian authorities have detained Jehovah's Witnesses in the central region of Mordovia, police said on Thursday, as part of a crackdown which the UN warned was setting a "dangerous precedent".
Russia brands the US evangelical Christian movement a totalitarian sect and in 2017 designated it as an extremist organisation, ordering its dissolution in the country.
On Wednesday, a Russian court sentenced Dennis Christensen, a Danish Jehovah's Witness, to six years in prison for "extremism" - the first adherent of the movement to be sent to prison in Russia.
The same day, several Jehovah's Witnesses were detained in Mordovia, police said.
"The large-scale operation" was conducted by the FSB security service together with police and other law enforcement agencies, police said.
"An investigation is under way," police said in a statement.
The sentencing of Christensen provoked an outcry, with rights groups comparing Moscow's pressure on the group to the persecution of the Soviet era.
The UN on Thursday condemned what it called a "dangerous precedent" and criticised the "criminalisation of the right to religious freedom."
"The harsh sentence imposed on Christensen creates a dangerous precedent, and effectively criminalises the right to freedom of religion or belief," UN High Commissioner for Human Rights Michelle Bachelet said in a statement.
The UN called on Russian authorities to revise federal laws that too vaguely define extremism and also release those being held for exercising their rights to religion or belief.
Jehovah's Witnesses are a Christian denomination that originated in the United States in the late 19th century and preaches non-violence.
Since 2017, a Russian crackdown on believers has intensified, with more than 20 Jehovah's Witnesses in custody, awaiting trial on extremism charges, while 25 are under house arrest.
Christensen was given the lengthy prison term even though Russian president Vladimir Putin said in December that Jehovah's Witnesses should not be seen as terrorists.
Putin's spokesman Dmitry Peskov said Thursday the Kremlin could not comment on the court ruling but added that officials would be looking into the issue.
"This will be happening. This is a process. This is not an easy subject," he told reporters.
Asked whether the Jehovah's Witnesses were an extremist or religious organisation according to common sense, Peskov said: "We cannot be guided by the notion of common sense for state purposes."
Foreign governments have said they were concerned by Christensen's sentencing.
"We agree with president Putin that persecuting peaceful believers is utter nonsense, and call on Russia to respect freedom of religion," Andrea Kalan, a spokeswoman for the US embassy in Moscow, said on Twitter.