Mr Haider's car overturned a number of times, causing him serious head, chest and spinal injuries. He died on the way to hospital.
Mr Haider had reportedly been due to attend his mother's 90th birthday celebrations later in the day.
The Alliance for Austria's Future (BZO) was one of two right-wing parties that did better than expected in general elections last month, fuelling speculation of a possible role in a ruling coalition.
Mr Haider's deputy, Stefan Petzner, has been named his successor.
Austria's President Heinz Fischer said Mr Haider's death was a "human tragedy", while Chancellor Alfred Gusenbauer described him as someone who had shaped Austria's domestic and political landscape over decades.
On Saturday, residents of Klagenfurt left candles and messages of condolence outside the governor's office.
Mr Haider was a divisive figure, who gained notoriety after he became leader of the Freedom Party in 1986.
In 1991, his term as governor of the province of Carinthia was interrupted, after he made comments praising employment policies of Nazi Germany.
The wreckage of the car Mr Haider was driving
But he was re-elected in 1999 and 2003.
In 2000, the EU imposed sanctions against Austria in a protest over his party's role in government.
In 2005, Mr Haider left the Freedom Party and founded the Alliance, which scored its best result so far in elections last month, gaining 11% of the vote.
This was, however, well below the 27% that the Freedom Party won under his leadership in 1999 - a high mark in Mr Haider's electoral career at national level.
"With his passing, Austria has lost a great political figure," said Heinz-Christian Strache, who had taken over as leader of the Freedom Party after Mr Haider left.
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