Mr Nemtsov served twice as deputy prime minister under Boris Yeltsin
A prominent Kremlin critic who is running for mayor of Russia's Black Sea resort of Sochi has said assailants squirted ammonia in his face on Monday.
Boris Nemtsov said he was attacked by three men outside his campaign HQ. He apparently suffered no lasting harm.
Mr Nemtsov said he believed pro-Kremlin activists had carried out the attack in response to his criticism of plans to hold the 2014 Winter Olympics in Sochi.
He said it was physically not ready for the burden placed on it by the Games.
The Kremlin has not yet commented on Mr Nemtsov's claim that pro-Kremlin activists were involved.
Russian President Dmitry Medvedev visited Sochi later on Monday to host a meeting discussing preparations for the Olympics, during which he warned that work was being "mired in bureaucratic problems".
"The authorities that are in charge of the process are in constant talks with each other and normal work has not yet started," he said.
Mr Nemtsov, a native of Sochi, became one of Russia's most prominent politicians following the fall of the Soviet Union.
The Western-orientated market reformer served twice as deputy prime minister under former Russian President Boris Yeltsin in the 1990s. However, he lost the job following the 1998 economic crisis and became a vocal critic of Vladimir Putin, now prime minister, during his eight-year presidency.
Speaking on Ekho Moskvy radio after the attack, Mr Nemtsov blamed activists from a pro-Kremlin youth group, Nashi.
"The authorities, who are obviously in a hysterical state, decided to resort to criminal elements, specifically members of the Nashi movement, judging by their style, by their faces - we recognised several of them," he said.
"And they - meanly, from behind a corner as always - jumped at us and squirted us with ammonium chloride."
He added: "Fortunately, I managed to rinse my eyes fast and generally recover myself rather quickly."
Mr Nemtsov said he had called the police three times afterwards but that no officers had visited the scene of the attack, adding: "They are terribly afraid."
A Kremlin spokesman said he could not immediately comment on the allegations. Nashi representatives have also not yet commented.
Two weeks ago, Andrei Lugovoi, the prime suspect in the murder in London in 2006 of Kremlin critic Alexander Litvinenko, announced that he would run for mayor of Sochi in next month's election.