Dmitry Medvedev, Russia's likely next president, portrayed himself as a modernising moderate as he launched his campaign on Tuesday for the March 2 poll.
However, Mikhail Kasyanov, his only liberal opposition challenger, saw his candidacy thrown into doubt by a criminal investigation into whether his campaign had forged signatures in his support.
Mr Medvedev, Vladimir Putin's preferred successor, told a gathering of Russia's political and intellectual elite in Moscow that there should be more respect for the law. He pledged to fight corruption, an area in which Mr Putin, who is constitutionally barred from the election, has made little progress in his eight years in office.
"Corruption in official structures is on a huge scale and the fight against it should be a national programme," he said.
Mr Medvedev, first deputy prime minister and the chairman of Gazprom, the huge energy company, steered clear of the aggressive rhetoric favoured by Mr Putin, insisting that Russia would continue to develop as a "country open for dialogue and co-operation with the international community".
But he emphasised continuity. "We simply need 10 years of stable development, which our country was deprived of for a long time," he said, reiterating the Kremlin's goal of turning Russia into one of the world's five leading economies within 15 years.
Mr Medvedev emphasised the importance of the development of an "open civil society" and functioning political institutions.
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But in a pointer towards what critics say will be a presidential election as heavily stage-managed as parliamentary polls last month, Mr Kasyanov, a former prime minister and now independent opposition candidate, found himself facing an official attack that could push him out of the race. Russian prosecutors launched a criminal investigation into accusations that Mr Kasyanov's campaign had forged a significant proportion of the 2m signatures he had to collect in support of his presidential challenge. If the allegations are upheld, he could be disqualified. Prosecutors said they found more than 15,000 faked signatures from two regions.
The new 2m signature requirement - described by critics as draconian - applies only to candidates who are not members of or backed by parties represented in Russia's parliament.
Mr Kasyanov's campaign said the forgery claims amounted to "political pressure". The leader of the liberal People's Democratic Union, who was sacked by Mr Putin as prime minister in 2004, had accused the authorities of conducting a "co-ordinated campaign of pressure" against him, including "crude intimidation" against some of his campaign workers.