Yulia Navalnaya, the wife of jailed Kremlin critic Alexei Navalny, has been detained by police along with hundreds of other protesters in Russia, she says.
Police have declared the rallies in Moscow and dozens of other cities on Saturday illegal and have arrested more than 800 people, the OVD-Info protest monitor said.
Navalny had called on his supporters to protest after being arrested last weekend as he returned to Moscow from Germany for the first time since being poisoned with a military-grade nerve agent in August.
Among those arrested was his wife Yulia, she wrote on her Instagram account from inside a police van.
In Moscow, at least 40,000 people were estimated to have attended the rally on Saturday in support of Navalny, calling for his release.
Authorities have said the protests are illegal because they had not been properly authorised.
Navalny was remanded in custody for 30 days earlier this week for alleged parole violations.
The Kremlin has not commented on the protests.
Mobile phone and internet services suffered outages on Saturday, the monitoring site downdetector.ru showed, a tactic sometimes used by authorities to make it harder for protesters to communicate among themselves and share video footage online.
Navalny, an ex-lawyer who has accused Putin of ordering his murder, could face years in jail over legal cases he calls trumped-up.
Putin has denied involvement in the poisoning.
The West has told Moscow to let Navalny go, sparking new tensions in already strained Russia ties as US President Joe Biden launches his administration.