Russian authorities have cracked down on criticism of the war in Ukraine, arresting thousands of protesters since the beginning of the conflict in February.
"We are deeply disturbed by the large number of people who have reportedly been arrested," United Nations rights office spokeswoman Ravina Shamdasani told reporters in Geneva.
Arrests have sky-rocketed since Putin announced last Wednesday a partial military mobilization to bolster troops in Ukraine.
Shamdasani pointed to "credible reports some 2,377 demonstrators had been arrested... in various locations across the country."
"It is unclear how many people remain in detention," she said.
On Saturday, police monitoring group OVD-Info counted at least 726 people in detention in 32 cities across Russia, nearly half of them in Moscow.
Shamdasani highlighted in particular the two days of protests in Russia's southern republic of Dagestan, where clashes erupted between demonstrators and the police.
"Dozens of people were reported to have been arrested," she said.
Dagestan — a poor, Muslim-majority republic in the North Caucasus — has seen more men killed in the Kremlin's military offensive in Ukraine than any other part of Russia, according to a tally made by independent Russian media of death notices published online.
In several regions, military and administrative buildings, including enlistment offices, have been attacked during anti-draft protests.
On Monday, a man opened fire and wounded a recruitment officer at an enlistment centre in Siberia.
But Shamdasani stressed that the majority of protests across Russia to date had reportedly been peaceful.
"We stress that arresting people solely for exercising their rights of peaceful assembly and freedom of expression constitutes an arbitrary deprivation of liberty," she said.
"We call for the immediate release of all those arbitrarily detained and for the authorities to abide by their international obligations to respect and ensure the rights to freedom of expression and of peaceful assembly."