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As much as $30bn (£20bn) has been stolen in preparations for next year's Winter Olympics in Sochi, Russian opposition leaders say.

Boris Nemtsov and Leonid Martynyuk say the games are "a monstrous scam" and will be the most expensive in history, costing more than $50bn.

A lack of fair competition and secrecy sharply increased the cost, benefitting only businessmen close to President Vladimir Putin, they say in a report.

A Russian official dismissed the claim.

Mr Nemtsov was quoted by Reuters as saying that he would now be pushing for a thorough investigation into the matter by Russian prosecutors.

He also questioned the wisdom of staging the games in a subtropical climate in the city on the Black Sea coast.

But Deputy Prime Minister Dmitry Kozak, who is responsible for the 2014 Games, was quoted as saying: "In spite of envious people, the Russian Winter Olympic Games will be held in the southern capital - subtropical Sochi."

Meanwhile, International Olympic Committee (IOC) spokesman Mark Adams said it appeared that Russia was taking any corruption claims related to the Olympics seriously.

Mr Adams said Moscow was addressing the issue "pretty much head-on from the president down", according to the Associated Press news agency.

Mr Nemtsov, a former deputy prime minister, and Mr Martynyuk, a member of the Solidarity movement, released their report on Thursday after analysing Olympic-related data for six months.

The authors say they reached their conclusions by looking at the gap between the original estimate and the final cost and also comparing it with cost overruns at previous Olympics.

"The expenses for the Winter Olympics in Sochi turned out to be more than all expenses for all the sports structures at the previous 21 Winter Olympics put together," the report says.

It says that "between $25bn and $30bn have been stolen", and this has benefited "only oligarchs and companies close to Mr Putin".

The opposition leaders claim that the stolen funds could have been used to build about 3,000km (1870 miles) of high-speed motorways or provide the housing for 800,000 people in Russia.

They say that the final cost of staging the event will be more than four times higher than the original price tag of $12bn announced by Mr Putin.

Details of their findings were released on the day Mr Putin met International Olympic Committee (IOC) officials in St Petersburg to unveil the medals for the Sochi games.