Warsaw Pact troops invaded what was then Czechoslovakia in early on August 21, 1968, taking control of the country and arresting the reformist leaders. Western nations denounced the crackdown but made no military move to stop it.

In the run-up to the emotional anniversary, Czech leaders and European commentators have drawn comparisons between 1968 and Russia's invasion of Georgia. So did US Secretary of State Condoleezza Rice.

Czech Prime Minister Mirek Topolanek said, ''The Russian tanks on the streets of Georgian towns remind those of us who experienced it of the 1968 invasion''.

At an anniversary event in neighbouring Austria, Foreign Minister Ursula Plassnik lashed out at Russia for its assault on Georgia.

''A power of the future cannot rely primarily on tanks for its foreign policy,'' she said.

''We never want to have to fear Russia again.''

German Chancellor Angela Merkel, who grew up in her country's formerly communist east, has urged people not to forget the ''brutal crushing'' of the Prague Spring.

Czech and Slovak government leaders the two parts of the country split peacefully in 1993 planned to mark the anniversary at events in both capitals, Prague and Bratislava.

Prague's National Museum was opening an exhibit with mementos and recollections from ordinary citizens. An old Soviet tank was parked outside the building as part of the show.

Also in the show are stories and personal items from Jan Palach, a Prague student who set himself on fire in January 1969 to protest against the invasion. He died a few days later, making him a martyr of the fight against communism.

However, a recent poll found that 70 per cent of Czechs younger than 20 had ''no opinion'' on the events of 1968.

Among Prague's expected guests were former dissidents from Russia, Bulgaria, Poland, Hungary and East Germany who protested against the invasion in their home countries.