Russian opposition politician Alexei Navalny on Thursday accused Prime Minister Dmitry Medvedev of controlling a property empire including mansions, yachts and vineyards financed by oligarchs through a network of shadowy non-profit organisations.
Medvedev, a lawyer from Saint Petersburg, served as president from 2008 to 2012 while Vladimir Putin served as premier between presidential terms.
Initially seen as a liberal-leaning reformer, Medvedev has made little public impact since he headed the government, although he heads the ruling United Russia party.
Navalny came to fame at mass protest rallies against Putin's 2012 return to the Kremlin. He heads a team of corruption busters that has released damaging revelations on top officials.
While Medvedev is widely seen as a harmless figure who frequently dozes off and loves gadgets, Navalny insists this is an illusion.
"Medvedev can steal so much and so openly because Putin does the same, only on a bigger scale," he wrote, presenting his team's online report.
Medvedev "practically openly created a corrupt network of charitable foundations through which he receives bribes from oligarchs and frantically builds himself palaces and vacation homes across the whole country," the report alleges.
Medvedev's spokeswoman dismissed the allegations as promotion for Navalny's vaunted presidential bid.
"Navalny's material is clearly electioneering in nature," Natalya Timakova told RIA Novosti state news agency.
"It's pointless to comment on the propagandistic attacks of an oppositional convict," she added.
Navalny has announced he wants to stand as a candidate for president against Putin in 2018 but faces legal hurdles.
He was re-convicted of fraud this year in a case involving a timber deal in the central Kirov region, and given a five-year suspended sentence.
The European Court of Human Rights had ruled his previous trial was unfair, but Russia responded by repeating the verdict word-for-word.
His anti-corruption foundation has made allegations of massive graft against Prosecutor-General Yury Chaika in a report viewed more than five million times.
The Medvedev investigation is the biggest yet, Navalny said.
The YouTube video had been viewed more than 140,000 times by Thursday afternoon.
The report alleges that Medvedev's former university classmates, friends and a relative manage the assets.
These include a luxury apartment block in Saint Petersburg with glass-walled lifts for cars so owners can view them in their living rooms.
"All this property was bought with oligarchs' bribes and state bank loans," the report says.
The property empire is "all serviced by a single centre. Financing is also centralised," the report alleges, saying apparently unconnected foundations have shared staff or linked Internet domains.
"The only element that unites all this into a system is Dmitry Medvedev," it concludes.