MOSCOW - The Kremlin faced a new crisis Monday after the opposition accused authorities of rigging a key regional election and protesters took to the streets to denounce the official results.

More than 2,000 people gathered in front of local authority offices in the far eastern city of Vladivostok after an opposition candidate suddenly lost a gubernatorial election to a candidate of the ruling United Russia, the Communist Party said.

Sunday's vote was a second-round runoff after the Moscow-backed candidate, Andrei Tarasenko, failed to win 50 percent of the vote. Russians had registered their anger over a controversial pension reform and deteriorating economic conditions under Western sanctions. Communist Party candidate Andrei Ishchenko seemed poised to become the next governor of the far eastern region of Primorsky Krai until the results changed significantly overnight, he and his supporters say.

With more than 95 percent of votes counted, Ishchenko had a five-point lead in the vote Sunday.

Then, overnight, Tarasenko overtook him to claim victory Monday.

Opposition leaders claimed officials rigged the vote after Putin voiced his support for acting governor Tarasenko in a meeting last week.

"I know you face a second-round runoff," Putin said at the time. "I think everything will be alright."

Ishchenko said he would not give up and the Communist Party would take legal action.

"Friends, they are stealing our votes, let's stand up for our victory," he said in a Facebook post, urging supporters to take to the streets.

He claimed it had taken the authorities just "one hour" to rig the polls by giving Tarasenko 40,000 additional votes. He said the most blatant violations had taken place in the towns of Ussuriisk, Artyom and Nakhodka.

Putin's most vocal critic, Alexei Navalny, also urged local residents to come out and protest.

"This is simply unbearable," said Navalny, who is serving a month-long jail term over an illegal protest.

"The open and impudent falsification of the results of the vote in Primorsky Krai is a direct insult not only to residents of this region but also the entire country."

Earlier this month, the Kremlin suffered several other blows in the regions of Khabarovsk, Khakasia and Vladimir where ruling party candidates failed to win gubernatorial elections in the first round.

The second-round runoffs are scheduled to take place in those regions on Sunday.