The G8 summit of big industrial nations is set to open in Germany amid a growing row over a US defence system.
Mr Bush accused Russia of slipping on democratic reform
US President George W Bush and his Russian counterpart, Vladimir Putin, have traded increasingly heated threats and accusations ahead of the meeting.
Mr Putin warns Russia will aim weapons at Europe if the US builds the shield as planned in Poland and the Czech Republic, former Soviet satellites.
UK leader Tony Blair says there must be a "frank discussion" about this threat.
The contentious issue of climate change is also up for debate at the summit, but is presently overshadowed by the increasingly fractious relations between Moscow and Washington.
Leaders are hoping to strike a deal to tackle climate change which includes both China and the US, which has opposed mandatory targets.
BBC diplomatic correspondent James Robbins says meetings of the world's most powerful leaders are always difficult, but this one looks especially tough.
The Czech Republic is the planned site for a radar base that will form part of the US missile defence shield, while interceptor missiles are due to be deployed in Poland.
Mr Bush, who will see Mr Putin in Germany, said the new system was a "purely defensive measure, aimed not at Russia but at true threats".
Speaking in the Czech Republic on the eve of the summit, the American president said the Cold War was over and Russia was not an enemy of the US.
But he went on to accuse Russia of sliding backwards on democracy.
"Reforms that once promised to empower citizens have been derailed, with troubling implications for democratic development," he said.
For his part Mr Putin has scoffed at US claims that the shield is designed to counter threats from states such as North Korea and Iran, indicating that the real target was Russia.
UK Prime Minister Tony Blair entered the debate in a BBC interview on Wednesday, saying it was not in Russia's interest to have a "scratchy" relationship with western countries.
London currently has its own issues with Moscow over the murder of former KGB agent Alexander Litvinenko.
Mr Putin has dismissed a UK extradition request for a Russian suspect as "pure foolishness".
Pushing at frontiers
Other issues on the agenda of the summit, which is taking place in the German Baltic resort of Heiligendamm, include the Middle East and the future of Kosovo, as well as the thorny subject of climate change.
European countries want a new treaty to replace the Kyoto accord with stricter limits on greenhouse gas emissions. The US, which has not ratified the Kyoto Treaty, is opposed to mandatory targets.
German Chancellor Angela Merkel must decide how far the most contentious issues can be pushed, says our correspondent.
Meanwhile, the German authorities are on alert to prevent disturbances at the summit.
As Mr Bush arrived at Rostock in Germany, hundreds of protesters gathered near the airport shouting: "President Bush is not welcome here."
During the weekend German riot police clashed with anti-globalisation protestors in the city. About 1,000 people were injured.
President Bush was quickly taken by helicopter to the summit venue.
The crowd soon dispersed and the latest demonstration passed off peacefully.