Georgia, a strategically-located republic at the center of the Caucasus, plunged into political crisis on Sunday as the ruling party celebrated victory in a pivotal election that the pro-Western opposition declared as falsified, vowing to boycott the new parliament.

The shaky situation further polarized a political struggle between Georgian Dream, which has governed Georgia for 12 years, increasingly steering it away from its decades-long path to join NATO and the European Union, and the four political groups that aimed to keep it moving toward the West.

The crisis will likely push Georgia further away from the West, with European observers criticizing the conduct of the election on Sunday. Zlatko Vujovic, the head of the European Network of Election Monitoring Organizations, characterized the election as “not good.”

“They were not conducted in the proper way as should be expected from a country that has a E.U. candidate status,” Mr. Vujovic said at a briefing with journalists.

Critical violations included violence against opposition members, voter intimidation, smear campaigns targeting observers, and extensive misuse of administrative resources, he said.

The Election Administration of Georgia, the body that oversees elections in the country, reported on Sunday that the ruling party garnered more than 54 percent of the vote with the four main opposition groups receiving 37.5 percent. That means that Georgian Dream will have 89 seats in Parliament with the remaining 61 occupied by the opposition.

But all four opposition groups said they did not recognize the election as legitimate, citing significant violations. At least two said they will not enter Parliament, setting the stage for a prolonged political crisis. They did not provide evidence for their allegations.

“Bidzina has conducted a K.G.B.-style special operation,” said Mamukha Khazaradze, leader of Strong Georgia, one of the opposition groups, in a post on Facebook, referring to Bidzina Ivanishvili, the informal leader of Georgian Dream. “The elections were totally rigged, the people’s vote was stolen and the country is being prepared for Russia.”

The previous parliamentary elections ended with a similar scenario, when the opposition refused to accept the official results. The crisis lasted more than five months before the ruling party and the opposition reached an agreement that was mediated by officials from the European Union.

But this time, the ruling party seemed determined to press ahead with what it said was a mandate from the people, and it is unlikely to welcome European mediation.

Speaking at a briefing on Sunday, Georgia’s prime minister, Irakli Kobakhidze, said that the election was administered “flawlessly.”

“Our victory is impressive and clear — any attempt to talk about manipulations in the context of electronic vote and unprecedented campaign circumstances is bound to fail,” said Mr. Kobakhidze, adding that the new parliament will operate regardless of whether opposition lawmakers enter it or not.

Leaders of Armenia and Azerbaijan, Georgia’s neighbors in the South Caucasus, congratulated Georgian Dream. Russian lawmakers and officials also commended the result.