Russian prosecutors have opened a criminal case against former PM Mikhail Kasyanov - an opposition candidate in the 2 March presidential election.
Mr Kasyanov had said the Kremlin was trying to sabotage his bid
They accuse his campaign of forging some of the reported two million signatures on his nomination papers.
Mr Kasyanov is challenging Mr Putin's candidate Dmitry Medvedev. Mr Putin himself is barred from the March poll.
A spokeswoman for Mr Kasyanov's campaign described the prosecutors' move as "political pressure".
On Saturday he issued a statement complaining of official harassment.
Mr Kasyanov was dismissed as prime minister in 2004, and the BBC's James Rodgers, reporting from Moscow, says his political journey has turned him from a loyal member of the Putin administration to an implacable critic of the Kremlin.
The authorities refused to register his political party, which is why he was obliged to support his candidacy with a list of signatures.
But prosecutors say they suspect more than 15,000 of these are fakes.
And the electoral commission says it has found 62,000 forged signatures.
If these figures were officially confirmed, they would disqualify Mr Kasyanov from standing.
Mr Kasyanov responded by saying "the authorities are afraid of a direct political contest".
"The question today is, will the voters have a choice or not?" he told reporters.
Mr Putin is standing down as president because he is not allowed more than two consecutive terms as president.
President Putin wants to continue working with Dmitry Medvedev
But he is widely expected to retain his influence in the Kremlin. He says he would like to be prime minister under the presidency of his protégé Mr Medvedev, who is hot favourite to win.
Two other candidates have also registered for the poll - Communist leader Gennady Zyuganov and Vladimir Zhirinovsky of the nationalist Liberal Democratic Party.
A leading liberal, former chess champion Garry Kasparov, had planned to run but said he could not after his supporters were not allowed to rent halls for nomination meetings.
The final candidates list is to be announced on Sunday.
Western observers have urged the Kremlin to stand back from the presidential poll.
They accused it of interfering in parliamentary elections last year, even though Mr Putin's party was likely to have won anyway.