Police in Moscow detained two women and five children who wanted to lay flowers at the Ukrainian embassy.
Photographs of their detention showed the children holding a poster saying “No to War.”
The children, aged 7 to 11, were held with their mothers in a police van before being taken to a police station. They were released hours later, according to anthropologist Alexandra Arkhipova.
Arkhipova said that the two women detained were Ekaterina Zavizion and Olga Alter.
The anthropologist, who works at the Russian Presidential Academy of National Economy and Public Administration, wrote on Facebook: “None of what’s happened is holding up in my head.”
Video footage on social media apparently showed one of the women explaining to a crying girl from inside a cell that the “task is for fewer people to gather and say they’re against the war”.
Arkhipova said that police allegedly threatened to strip the women of custody over the five children. They face a trial and a fine on unspecified charges, Arkhipova added. She wrote: “The parents are in fear.”
Another sign held by one of the children featured a Russian flag followed by a plus sign and a Ukrainian flag, equalling a heart.
The OVD-Info website, which monitors protests and arrests across Russia, said that children up to the age of 14 cannot be legally held for more than three hours.
The Ukrainian foreign minister, Dmytro Kuleba, who shared images of the children, said it was another sign of the toll that Putin’s war against Ukraine is taking on children.
He wrote on Twitter: “Putin is at war with children. In Ukraine, where his missiles hit kindergartens and orphanages, and also in Russia. [These children] spent this night behind bars in Moscow for their ‘NO TO WAR’ posters.
“This is how scared the man is.”
A 77-year-old artist and activist, Yelena Osipova, was also marched away by a group of police while she protested against the war in St Petersburg.
Thousands of people in cities across Russia have been defying police threats and staging protests against the invasion of Ukraine. Authorities have a low tolerance for demonstrations and marches, and attending them can have serious consequences including fines, arrests and even imprisonment.
OVD-Info said that authorities have arrested more than 320 anti-war protesters across 33 Russian cities. A total of 6,840 people have been detained since Vladimir Putin, the Russian president, ordered the invasion of Ukraine last Thursday.
