Vaclav Havel, the Czech Republic's first president after the Velvet Revolution against communist rule, has died at the age of 75.
The former dissident playwright, who suffered from prolonged ill-health, died on Sunday morning, his secretary Sabina Tancecova said.
As president, he presided over Czechoslovakia's transition to democracy and a free-market economy.
He oversaw its peaceful 1993 split into the Czech Republic and Slovakia.
Havel first came to international fame as a dissident playwright in the 1970s through his involvement with the human rights manifesto Charter 77.
A black flag has been flying over Prague Castle, the presidential seat, and people have been gathering in Wenceslas Square, scene of anti-communist protests in 1989, to light candles in honour of Havel.
The Czech cabinet is to meet for a special session on Monday to consider arrangements for national mourning.
Tributes have been pouring in for the man many consider a driving force in the overthrow of communist rule in eastern Europe.
"His peaceful resistance shook the foundations of an empire, exposed the emptiness of a repressive ideology, and proved that moral leadership is more powerful than any weapon," said US President Barack Obama.
German Chancellor Angela Merkel hailed Havel as a "great European" in a letter of condolence to Czech President Vaclav Klaus.
"His fight for freedom and democracy was as unforgettable as his great humanity," wrote Mrs Merkel, who grew up in communist East Germany.
"We Germans in particular have much for which we are grateful to him. We mourn this loss of a great European with you," she wrote.
British Prime Minister David Cameron said he was "deeply saddened" and that Europe owed Havel a "profound debt".
"Havel devoted his life to the cause of human freedom. For years, Communism tried to crush him, and to extinguish his voice. But Havel could not be silenced.
Swedish Foreign Minister Carl Bildt wrote on Twitter: "Vaclav Havel was one of the greatest Europeans of our age. His voice for freedom paved way for a Europe whole and free."
Havel died at his country home north-east of Prague.
In his final moments, he was comforted by his wife Dagmar and several nuns, his secretary was quoted as saying by Reuters news agency.
Havel had looked thin and drawn during recent public appearances.
A former heavy smoker, Havel had a history of chronic respiratory problems dating back to his years in communist prisons.
He had part of a lung removed during surgery for cancer in the 1990s.
He was taken to hospital in Prague on 12 January 2009, with an unspecified inflammation, and developed breathing difficulties after undergoing minor throat surgery.
Havel began co-writing plays during his military service in the 1950s and his first solo play, The Garden Party, was staged in 1963.
His plays satirised the absurdities of life under communist rule, but his work was banned after the reformist Prague Spring of 1968 was crushed by a Soviet-led invasion.
After that his plays were banned and he was imprisoned several times.
By the late 1970s he had become Czechoslovakia's best-known dissident. He helped found the Charter 77 movement for democratic change.
When communist rule unravelled in late 1989, he was elected president by the interim coalition cabinet. He resigned in 1992 after Slovak nationalists successfully campaigned for the break-up of Czechoslovakia.
He was elected first president of the Czech Republic in January 1993, serving until 2003 when he resigned as his health deteriorated.
Havel returned to literature and to supporting human rights activists around the world.