MOSCOW — Major opposition parties in the South Caucasus nation of Georgia vowed on Wednesday to boycott Parliament until the government releases a prominent opponent detained recently.

The instability adds yet another country to a growing list of former Soviet republics gripped by political tensions, street protests or outright war.

Just in the past few months, demonstrations have shaken the government in Belarus, Kyrgyzstan has endured its third post-Soviet revolution, and Azerbaijan and Armenia have fought a vicious war over a breakaway enclave.

Though politics in Georgia, a country of just over four million people, have always been sharp-elbowed, the arrest of the opposition leader, Nika Melia, suggested an alarming pivot to more repressive policies by the governing party, Georgian Dream.

Mr. Melia, chairman of the United National Movement, a political party founded by a former president, Mikheil Saakashvili, had blockaded himself into the party’s headquarters in Tbilisi, the capital. To make the arrest, police officers scaled fire ladders onto the roof and battered through barricades of furniture inside the building.

Mr. Melia stands accused of fomenting a crowd to storm Parliament in 2019, a charge he has dismissed as politically motivated.

In a joint statement issued on Tuesday, several United States senators sharply criticized the arrest, saying it “jeopardizes what remains of Georgia’s democracy and its Euro-Atlantic path.”

The statement called for Mr. Melia’s release and for a dialogue between parties to resolve the political crisis that has been brewing since a contested election in October. Members of several opposition parties, including the United National Movement, contend that the vote was rigged and have refused to be seated in Parliament. They have vowed to continue the boycott until Mr. Melia is released.

A member of the United National Movement, Zaal Udumashvili, told local news outlets, “We are ready to sit down at the negotiating table, provided that Nika Melia will also be sitting at the table.” Several thousand people protested Mr. Melia’s arrest in central Tbilisi on Tuesday evening.

Underlying the political crisis are accusations from the opposition that a billionaire who went into politics, Bidzina Ivanishvili, a backer of the governing party, has destroyed the country’s pluralistic institutions, something Mr. Ivanishvili denies.

Shota Utiashvili, vice president of the Atlantic Council of Georgia, said in a telephone interview, “Georgia has been labeled as a beacon of democracy in the region, and it’s really unfortunate to see it sliding toward these signs of authoritarianism.”

“Georgia has never been a perfect democracy, but at least its trajectory was in the right direction,” he added.

The arrest has also roiled Georgian Dream, the governing party. The prime minister, Giorgi Gakharia, a member of the party, resigned last week to protest the issuing of a warrant for Mr. Melia’s detention. “Polarization and confrontation pose the greatest risks to our country’s future,” he said.

The escalating standoff over the disputed election has alarmed Western diplomats who for years have held up Georgia as a democratic success story in the former Soviet Union.

The State Department issued a statement last week saying it was “deeply concerned” about the political parties’ inability to resolve the election dispute. The United States, it said, called “on all parties to exercise restraint and avoid any actions or rhetoric that could escalate tensions or result in violence.”