MOSCOW — Georgian authorities arrested the South Caucasus country's top opposition leader Tuesday, deepening its political rift and plunging yet another ex-Soviet state into crisis.

Nika Melia, leader of the United National Movement, Georgia’s main opposition party, was dragged from his party headquarters early Tuesday in a scene broadcast on live television. His party posted photos on Twitter of a large contingent of riot police entering the building and said the security forces used tear gas to arrest dozens of Melia’s supporters inside.

The unrest is the latest upheaval along Russia’s vast borders: Protests continue in Belarus over an August presidential election result that the opposition has denounced as fraudulent, and Kyrgyzstan recently had its third revolution in the past 15 years.

Georgia, a country of about 3.7 million people bordering the Black Sea, was considered more democratic than the other two, with ambitions to join NATO. But Tuesday’s escalation could ultimately work to Moscow’s benefit, potentially alienating Tbilisi from its powerful Western allies. In recent years, U.S. lawmakers have expressed concerns in letters to Georgia’s leadership that some actions by the ruling Georgian Dream party indicated a “backsliding.”

Billionaire oligarch Bidzina Ivanishvili, the founder of Georgian Dream, announced his exit from Georgian politics last month, but analysts have said he is probably still calling the shots behind the scenes.

The political tension started in October, when the Georgian Dream coalition again swept to victory, but opposition groups claim that the voting was rigged and refused to take their seats in Parliament. Critics have accused Georgian Dream of consolidating near-total control over all branches and levels of government.

The United National Movement party, founded by Mikheil Saakashvili, Georgia’s former president now living in exile in Ukraine, won just 27 percent of the vote. Melia was named the party’s new leader in late December.

The Organization for Security and Cooperation in Europe concluded that the October vote in Georgia was “competitive and, overall, fundamental freedoms were respected,” but it also cited “pervasive allegations of pressure on voters and blurring of the line between the ruling party and the state.”

Then last week, Prime Minister Giorgi Gakharia suddenly resigned from his post, citing the plans to arrest Melia as his reason for stepping down.

The 41-year-old Melia has said the charges against him of “organizing mass violence” during anti-government protests in 2019 are politically motivated. He faces nine years in prison if convicted.

“I’d like to believe that this step will help de-escalate the polarization on the political stage of our nation,” Gakharia said of his resignation in a news conference Thursday, adding that the decision to place Melia in pretrial detention posed “a risk to the health and lives of our citizens.”

In the wake of Gakharia’s surprise resignation, the Interior Ministry said Melia’s arrest was “temporarily postponed.” But less than 24 hours after Irakli Garibashvili was approved as the new prime minister by the Parliament on Monday, Melia was seen being led out of his party offices and into a black unmarked SUV by Georgian police.

In a statement, the U.S. Embassy in Georgia said it is “dismayed by the polarizing rhetoric from Georgia’s leadership at a time of crisis.” It added that “today, Georgia has moved backward on its path toward becoming a stronger democracy in the Euro-Atlantic family of nations.”

“Shocked by the scenes at UNM headquarters this morning,” British Ambassador to Georgia Mark Clayton wrote on Twitter. “Violence and chaos in Tbilisi are the last thing Georgia needs right now. I urge all sides to act with restraint, now and in the coming days.”