Mikhail Kasyanov, whom President Putin fired in a government purge in February, was due to speak on Saturday in Kursk, near the Ukrainian border, but was blocked by activists from Nashi, a pro-Putin youth group.
Tatyana Razbash, Kasyanov's spokesman, told the Interfax news agency: "Kasyanov was due to speak at [a] conference, but before it started hundreds of young people - who made no secret of the fact that they were members of the Nashi ["Ours"] movement - surrounded the building."
She said that Kasyanov, whom Putin replaced as prime minister with Mikhail Fradkov, made no attempt to break through the crowd "so as not to provoke it".
He entered the building two hours later, after the activists had dispersed.
'Everything is possible'
Ivan Mostovich, a spokesman for Nashi, said the protest was an initiative taken by the group's Kursk branch, but added: "I think there will be similar actions in other cities where Kasyanov tries to deliver speeches."
Kasyanov said on Tuesday that he planned to found a new democratic coalition, following the example of liberal parties in Moscow city council elections this month.
He is reported to have close ties to the oligarchs who ruled Russia under Boris Yeltsin, the former president. In February, when asked if he planned to run for president, he said: "Everything is possible."