Russian prosecutors accused Alexei Navalny of receiving funds from more than 300 foreign individuals and corporations and anonymous donations from 46 countries sent through 347 IP addresses, RIA Novosti reported.
Navalny ridiculed the charges in a blog post, saying a foreign IP address does not mean a foreign citizen made the donation.
He added that all of his campaign donations had been approved by Moscow's Election Commission or returned to donors.
The Prosecutor General's Office said information about Navalny's donations had come "via the Yandex-Money electronic payment system," ITAR-Tass reported.
The Internet payment processor said, "we do not understand within what parameters" the prosecutor's office made the charge. The company said it had not been contacted by prosecutors and noted that only individuals, not companies, can use its service.
Vladimir Zhirinovsky, the leader of the nationalist Liberal Democratic Party who initiated the complaint, said he could not tell whether donations to Navalny's campaign came from domestic or foreign supporters.
Incumbent Sergei Sobyanin is seen by most polls as the front-runner in the Sept. 8 election. He has raised $3 million to Navalny's $958,000, election commission records show.