Russian billionaire Mikhail Prokhorov said on Monday he will challenge Prime Minister Vladimir Putin in the presidential election next March.
Mr Prokhorov is ranked by Forbes as Russia's third richest man with a fortune of around $US18 billion.
In a recent blog, Mr Prokhorov said he saw no alternative to Mr Putin as president.
The BBC reports there was another demonstration in Moscow on Saturday by protesters calling for fresh parliamentary polls, claiming voting fraud in the elections on 4 December.
Mr Prokhorov said he would not build his presidential campaign on criticism of Mr Putin.
"Criticism must make up no more than 10% … I would like to focus on the things I would do," he said.