Karoui, smiling and waving, walked out of Mornaguia prison, 20 kilometres (12 miles) from Tunis, and was carried on supporters' shoulders to a waiting black limousine.

The release came shortly after lawyer Kamel Ben Messoud said the Court of Cassation had decided to free the 56-year-old business tycoon who has been behind bars since August in a money laundering probe.

Sunday's runoff pits Karoui against independent law academic Kais Saied, both political newcomers who emerged the winners of a September 15 first round.

Saied had announced last weekend that he was quitting campaigning to avoid an unfair advantage over Karoui, who had several previous appeals to be released turned down by the courts.