Russian prosecutors on Monday accused protest leader Alexei Navalny of breaking the law by receiving donations from foreign nationals, a claim he denied in a radio interview.
Navalny, 37, a charismatic lawyer and a star speaker at anti-Putin rallies, is running for Moscow mayor against a Kremlin-backed incumbent. He was dramatically released from prison last month to allow him to stand.
"A check has confirmed information about foreign financing of Alexei Navalny," the prosecutor general's office said in a statement published on its website.
Under Russian law, it is illegal for a candidate to spend money donated by foreigners on a political campaign.
The statement said 300 foreign nationals and anonymous donors based in countries including the United States gave Navalny money using a Russia-based Internet payment system.
Prosecutors said they had handed the information to the interior ministry, should it decide to open a criminal case.
Navalny, who is shaping up to be the main challenger to pro-Putin mayor Sergei Sobyanin in the election, denied the claim.
"All the payments that are transferred to our electoral account are checked by the city electoral commission. If they don't like them, we return those payments, and we do this quite often," Navalny told the Moscow Echo radio station.
"In this sense we have the most transparent campaign of anyone."
Navalny has conducted a highly professional campaign for mayor, taking to Moscow's streets to meet voters.
He was sentenced to five years in prison last month after being convicted of embezzlement in a widely disputed verdict. In a surprise decision, a higher court temporarily granted him freedom, allowing his mayoral campaign to go ahead.
The Moscow electoral commission's website said that Navalny had returned more than 2 million rubles ($61,000) worth of illegal donations as of last week.
He called the claim a tactic to ensure that Sobyanin wins at least 50 percent of the vote as a way of ensuring the election does not go into a second round.
"We understand that the more inevitable a second round becomes, the more the prosecutors and electoral commission dream up some rubbish to brainwash people on national television," Navalny said.
Asya Melkumova, spokeswoman of the Internet payment system Yandex Money, told Moscow Echo radio that prosecutors had not contacted the company.
She questioned why prosecutors talked about payments coming from IP addresses in foreign countries, saying this did not prove the nationality of users.
Sobyanin called the polls early, hoping to prove he had the support of Moscow's citizens, and said that Navalny should be allowed to stand.
But Navalny last week needled Sobyanin by calling for a probe into a large apartment owned by his teenage daughter, which he estimated was worth more than $5 million. Sobyanin's team said the mayor acquired it legitimately.
Sobyanin has refused to take part in televised debates with Navalny and other candidates, the first of which was due to be held Monday evening.
Navalny is predicted to win less than 10 percent of the vote in the September 8 election, while Sobyanin is forecast to gain more than half the votes required to give him an automatic victory, according to opinion polls.
Prosecutors launched the probe against Navalny at the request of nationalist politician Vladimir Zhirinovsky, whose Liberal Democratic Party of Russia generally backs the Kremlin and has a candidate standing in the race.