A judge ordered on Friday the detention of Tunisian presidential election candidate Nabil Karoui to face charges of tax evasion and money laundering, Mosaique FM radio reported.
Karoui's own Nessma TV channel reported he had been arrested as he travelled to Tunis.
The 56-year-old media magnate is one of the main candidates contesting the Sept 15 election following the death of President Beji Caid Essebsi.
A judge decided in July this year to bar Karoui from travelling abroad after weeks of investigation on suspicion of money laundering.
"Police arrested Karoui while we were on our way back from the city of Beja to Tunis," said Osama Khelifi, a political adviser to the candidate.
The election follows the death of Essebsi last month aged 92.
Tunisia's election commission has approved 26 candidates, including two women. They also include Prime Minister Youssef Chahed and former president Moncef Marzouki.