Dominik Duka, the archbishop of Prague, was to celebrate the funeral mass at St. Vitus Cathedral for Havel, who died Sunday at age 75, the BBC reported.
Thousands of mourners have been to Prague Castle, where Havel has been lying in state since Wednesday.
The funeral will be shown on large screens at Prague Castle and in Hradcany Square.
U.S. Secretary of State Hillary Clinton and her husband, former President Bill Clinton, U.K. Prime Minister David Cameron and the presidents of France and Germany are expected, as is Lech Walesa, the former dissident who became Poland's president, and Eastern European leaders.
"I wish that all who are not indifferent to the future of our country pursue their opinions with the same courage and conviction as Vaclav Havel did," said current Czech President Vaclav Klaus.
German Chancellor Angela Merkel, in a letter to Klaus, said Havel's "fight for freedom and democracy was as unforgettable as his great humanity."
"We Germans also have much to thank him for. Together with you, we mourn the loss of a great European," she said.
Havel, who had been a playwright in the 1960s, satirized the old-guard communism but his work was banned after the Soviet invasion of 1968, driving him underground. As a well-know dissident, he was jailed for "anti-state activity."
When communism fell in 1989, he made the transition from political prisoner to president in months. He resigned after Slovak nationalists won their 1992 campaign for independence, but the following year he was re-elected president and served until 2003.
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