Russia-Ukraine updates: Russian missile strikes hit multiple Ukrainian cities

In the wake of Saturday's short-lived attempted rebellion against the Kremlin by the Wagner private military company, Mikhail Khodorkovsky, an exiled oligarch considered one of Putin’s best-known opponents, told ABC News he supports the mutiny and encourages Russians to back the leader of the mercenary group.

Once Russia’s richest man, Khodorkovsky, a Putin opposition activist, spent 10 years imprisoned after he challenged Putin, his case now considered a foundational moment for Putin’s regime.

When Wagner founder Yevgeny Prigozhin and his fighters marched on Moscow Saturday before making a sudden about-face, Khodorkovsky was notable among Russia’s pro-democracy opposition in calling on people to support Prigozhin, arguing that allowing him to remove Putin would create an opportunity for the democrats.

Khodorkovsky told ABC News he believed Prigozhin’s actions were a real coup attempt and that it had “seriously undermined” Putin’s power. He predicted that similar opportunities to collapse the regime will be launched soon.

"The blow to Putin’s reputation, to the authorities’ reputation, was absolutely fantastical," Khodorkovsky said. "Putin’s government today is, without a doubt, strongly undermined by what happened -- his authority, his ability to control the security services is seriously undermined."

Khodorkovsky said Prigozhin’s march on Moscow had undermined Putin’s popularity, showing neither ordinary Russians nor the security services were prepared to act to protect him.

“Along the entire route of Wagner's columns, no one in any way tried to hinder him (Prigozhin). Even the security forces did not try to stop him," Khodorkovsky said. “It showed that, in fact, inside the country, Putin has an absolute void."

Khodorkovsky said he did not support Prigozhin himself -- considering him a "war criminal" -- but that the democratic opposition should have sought to help him overthrow Putin, and then taken power from him after.

Khodorkovsky criticized other parts of the anti-Kremlin opposition who attacked him for calling on people to assist Prigozhin, saying he believed the opposition had “slept through” the opportunity and suggesting it should have sought to stage a rebellion in Moscow at the same time.

"There will definitely now be more such opportunities because of Putin’s weakening. But the next time we need to simply be more ready," said Khodorkovsky, who is living in exile in England. "If an uprising had started in Moscow to meet Prigozhin then a situation could have developed quite differently.

ABC News' Patrick Reevell

Travel restrictions remained in place on Sunday on the major M-4 highway near Moscow, according to Russia’s state-run media.

"According to the previously issued regional decisions, traffic restrictions remain in force on the M-4 Don highway near the Moscow Region and the Tula Region," the federal road agency said, according to TASS.

The security checkpoints had been put in place Saturday as a column of Wagner Group forces traveled toward Moscow. Roadblocks in southern Russia, including in Rostov-on-Don and the Krasnodar Region, were reportedly lifted.

"All previously imposed restrictions on highways have been lifted," TASS reported, citing a branch of the Russian Interior Ministry.

-ABC News' KJ Edelman

Senior congressional leaders were briefed about the ongoing situation in Russia, according to a congressional aide.

U.S. intelligence officials told the so-called "Gang of Eight" -- the top Republicans and Democrats currently in congressional leadership-- in recent days about potentially concerning movements of Wagner Group forces and equipment build-ups near Russia. However, it was unclear to U.S. intelligence what was going to happen and when.

-ABC News' Trish Turner

A senior U.S. official says it is a mystery as to why Yevgeny Prigozhin stopped his march to Moscow given that he was seemingly in a dominant position.

The official said he was greeted as a hero in Rostov-on-Don. However, the senior official told ABC News that Prigozhin is in an "emotional state," and perhaps did it because he thought this would destroy Russia, or because he glimpsed his own end. It is impossible to tell whether Prigozhin thinks Russian President Vladimir Putin will actually honor their deal which included allowing Wagner group soldiers to be folded into the Russian military.

The official said that Belarus President Alexander Lukashenko was very effective, telling Prigozhin and Putin that this was all a misunderstanding and that they are both patriots and aligned for the same goals.

Putin is said to be completely shocked by how fast Wagner Group moved through Russia. For now, he is just trying to secure his position. He does not want to be seen negotiating over his defense minister, but the official says the U.S. believes concessions were made over Sergei Shoigu's future as well as others.

-ABC News' Martha Raddatz