Russia's central bank hiked its key interest rate to 20% on Monday in a desperate attempt to shore up the plummeting ruble and prevent a run on banks amid crippling Western sanctions over the Russian war in Ukraine.

The bank hiked the benchmark rate to 20% from 8.5%. That followed a Western decision Sunday to freeze Russia's hard currency reserves, an unprecedented move that could have devastating consequences for the country’s financial stability.

It was unclear exactly what share of Russia’s estimated $640 billion hard currency coffers would be paralyzed by the decision by Western nations to block Russian banks from the SWIFT global payment system. European officials said that at least half of it will be affected.

That dramatically raised pressure on the ruble by undermining the financial authorities’ ability to conduct hard currency interventions.

The ruble sank about 30% against the United States dollar early Monday but steadied after the central bank's move. It was trading at a record low of 105.27 per dollar, down from about 84 per dollar late Friday.

The rate hike is above the level of 17% seen in 2014 when Russia annexed Crimea from Ukraine.

Russia calls its actions in Ukraine a "special operation" that it says is not designed to occupy territory but to destroy its southern neighbor’s military capabilities and capture what it regards as dangerous nationalists.

"External conditions for the Russian economy have drastically changed," the central bank said in a statement.

Central bank chief Elvira Nabiullina will hold a briefing at 1 p.m. GMT, the bank said.

The recent moves add to a slew of measures announced since Thursday to support domestic markets, as the state scrambles to manage the broadening fallout from Western sanctions.

In another attempt to support the ruble, the central bank and the Finance Ministry also jointly ordered Russian exporting companies to sell 80% of their foreign currency revenues on the market.

Russian authorities have also ordered brokers to suspend short selling on the Russian market and stop executing orders by foreign legal entities and individuals to sell Russian securities.

"These measures may help calm down increased market nervousness, but at the same time they undermine the foundation of the monetary policy, focused on inflationary targeting and flexible exchange rate," BCS Global markets said in a note.

"Unfavorable external environment made Russia's monetary policy unsustainable and we do not rule out a possible rate hike going forward or further unexpected and non-market decisions."

The U.S., Japan and other Western nations moved over the weekend to impose additional sanctions against Russia, including restrictions on access for some Russian banks to the SWIFT global bank payments system.

The United Kingdom, meanwhile, on Monday unveiled new sanctions targeting Russia's central bank, following already announced ones aimed at imposing "severe consequences" on the Russian economy.

"The UK Government will immediately take all necessary steps to bring into effect restrictions to prohibit any UK natural or legal persons from undertaking financial transactions involving the Russian Central Bank, the Russian National Wealth Fund, and the Russian finance ministry," a government statement said.

"These measures demonstrate our determination to apply severe economic sanctions in response to Russia's invasion of Ukraine," Rishi Sunak, the chancellor of the exchequer, said.

This action is being taken "in rapid coordination with our US and European allies," he said.

Also commenting on the sanctions, Andrew Bailey, the governor of the Bank of England, said the bank continues to take any and all actions needed to support the government's response to Russia's military operation in Ukraine.

"This as an important and powerful demonstration of the UK's commitment to the international rule of law."

"More than 100 companies and oligarchs at the heart of (Russian President Vladimir) Putin's regime have been hit with sanctions worth 100s of billions of pounds, asset freezes and travel bans," according to the statement.

The sanctions will hit Russia's banking and defense sector hard-asset freezes on VTB, Russia's second largest bank, worth 154 billion pounds ($206 billion); Rostec, Russia's defense giant, responsible for $13 billion of arms exports per year, it said.

The sanctions will also soon be implemented on "571 members of the Duma and Federation council who sanctioned the invasion of Ukraine," the statement added.

The decline of the ruble would likely send inflation soaring, hurting all Russians and not just the Russian elites who were the targets of earlier sanctions. The resulting economic disruption, if Saturday's measures are as harsh as described, could leave Putin facing political unrest at home.

Analysts predicted intensifying runs on banks by Russians, and falling government reserves as Russians scrambled to sell their targeted currency for safer assets.

The SWIFT financial messaging system daily moves countless billions of dollars around more than 11,000 banks and other financial institutions around the world.

Allies on both sides of the Atlantic also considered the SWIFT option in 2014, when Russia invaded and annexed Ukraine's Crimea and backed separatist forces in eastern Ukraine. Russia declared then that kicking it out of SWIFT would be equivalent to a declaration of war and the allies shelved the idea. Russia since then has tried to develop its own financial transfer system, with limited success.

The disconnection from SWIFT announced Saturday was partial, leaving Europe and the United States room to escalate penalties later. Officials said they had not fully settled on which banks would be cut off and that the aim was for targeted, functional restrictions.