Russia has begun arresting anti-war protesters as demonstrations break out after Putin invades Ukraine

• The Russian government threatened anti-war protesters demonstrating against Russia's invasion of Ukraine.
• Russia's Investigative Committee said protesters face arrest.

The Russian government on Thursday threatened anti-war protesters demonstrating against Russia's invasion of Ukraine, warning they could face arrest for organizing.

And according to a protest monitoring group, the detentions have already begun as growing protests have broken out in some Russian cities.

Russia's Investigative Committee warned citizens in a statement not to take part in the "unauthorized" protests "associated with the tense foreign political situation."

The committee said that people should be aware of the "negative legal consequences of these actions," which it said includes criminal liability.

"The law provides for severe punishment for organizing mass riots, as well as for resisting law enforcement officers," the committee said.

It added that citizens who undertake "such illegal acts may face imprisonment."

According to protest-monitoring group OVD-Info, more than 960 anti-war demonstrators have been detained in 44 cities just before 9 p.m. local time.

Videos of protesting Russians taking to the streets in multiple cities, including in St. Petersburg and Novosibirsk, emerged less than a day after Russian President Vladimir Putin launched a war on Ukraine early Thursday.

One video posted by Russian news outlet Novaya Gazeta showed a group of people gathered in St. Petersburg with some being hauled away by authorities.

The crowd of roughly 200 demonstrators chanted, "No to war!" according to the news outlet.

Marina Litvinovich, a prominent human rights Moscow-based activist, who earlier Thursday called on Russians via social media to protest in multiple cities across the country, told Reuters that she was detained on her way out of her home.

Russia's conflict with Ukraine has been rumbling for years, but dramatically escalated in recent weeks.

Russia assembled vast numbers of troops around Ukraine — as many as 190,000, per US estimates — in the largest military operation in the region since World War II.

Putin on Monday recognized the independence of two Moscow-backed separatist regions in eastern Ukraine — Luhansk and Donetsk — and ordered troops there for what he described as a limited peacekeeping operation in the east of the country.

Less than 72 hours later, Putin authorized a full-scale attack on Ukraine. In the hours that followed, explosions pounded cities around Ukraine, many hundreds of miles from the previous conflict zone. Ukrainian officials reported fighting on its borders with Russia and dozens of casualties.

The new wave of hostilities expanded the clash from a limited incursion over disputed land into the most serious armed conflict in Europe in at least a decade.

Insider's live blog of the invasion is covering developments as they happen.