Senegal opposition leader Ousmane Sonko was arrested on Sunday while leading a convoy of supporters from his native Zighinchor, in Casamance, towards the capital, Dakar.
The arrest by security forces abruptly ended Sonko's cross-country march, which had begun two days earlier and was followed by police.
A man was killed in clashes that broke out between security forces and supporters of Sonko, who leads the Pastef party, which he founded in 2014.
"There was a death in Kolda," Interior Minister Antoine Diome said on Senegalese Radio and Television (RTS). "Will the state remain idly by? The answer can only be no."
Diome said Sonko should have requested prior authorisation before organising what he called a "freedom caravan".
He told RTS that Sonko had not been arrested, but rather "dropped off" at his Dakar home.
Pastef member Sekouna Diatta, Mayor of Mangagoulack, a rural town also in Casamance, told RFI people who had come to see the convoy had been fired on with tear gas by police.
"We denounce this with the greatest energy ... We're Senegalese citizens using a Senegalese public road," he said.
Rape charges and risk of ineligibility
Sonko is facing a rape trial, which could result in him being ineligible to run in next year's presidential election.
Last Wednesday, a prosecutor called for a 10-year prison sentence.
Sonko has been charged with rape and making death threats against an employee of a beauty salon in Dakar. Adji Sarr, 20, said she had been abused five times by Sonko.
A decision is expected to be handed down on 1 June.
Response to so-called political pressure
Sonko set off from Zighinchor for Dakar on Friday to attend the trial, which he has slammed as politically motivated.
He denies assault, saying he went to the salon for a massage for chronic back pain. He claims the charges against him are a scheme by President Macky Sall to disqualify him from the 2024 presidential election. Which Sall denies.
Sonko has documented his movements live on social media for the first two days of his trip, but he stopped posting on Sunday.
His Pastef party said it would "react to any attack on his physical integrity, his imprisonment or house arrest".
Sonko is especially popular among people aged under 20, who comprise half of Senegal's booming population.