The United Nations on Friday condemned numerous “arbitrary arrests” of people in Russia for protesting the country’s invasion of Ukraine and urged their immediate release.

“Arresting individuals for exercising their rights to freedom of expression or a peaceful assembly constitutes an arbitrary deprivation of liberty,” UN rights office spokeswoman Ravina Shamdasani told reporters in Geneva.

She said the agency understood that “more than 1,800 protesters were reportedly arrested. It is unclear whether some have now been released.”

Her comment came a day after Russian President Vladimir Putin defied Western warnings to unleash a full-scale ground invasion and air assault on Ukraine that quickly claimed dozens of lives and displaced at least 100,000 people.

Protesters turned out in cities around the world, including in Russia.

An independent monitor said police had detained protesters in 51 cities across Russia, cracking down on dissent after authorities warned citizens against marching.

“We call on the authorities to ensure the immediate release of all those arbitrarily detained for exercising these rights,” Shamdasani said.

UN rights chief Michelle Bachelet warned Thursday that the Russian invasion “clearly violates international law and puts at risk countless civilian lives.”

“It must immediately be halted,” she said.

She said her office would continue to closely monitor the situation and warned that “an information war is also underway.”

“It is particularly crucial at this time that we continue to closely monitor and attempt to verify reports of human rights violations, including civilian casualties, damage to civilian objects, including critical infrastructure, and other impact on human rights on the ground.”