Backers of detained opposition presidential candidate Bassirou Diomaye Faye are taking his campaign to the streets of Senegal with posters and bracelets, drumming up support in the hope he will be released before the vote.

Faye was relatively unknown within Senegal's tumultuous political scene until popular firebrand opposition leader Ousmane Sonko backed his candidacy for the March 24 vote.

Sonko, who enjoys widespread support among Senegal's jobless youths, is also in detention and disqualified from the race over a defamation conviction. Sonko denies any wrongdoing.

Faye, who is facing charges including defamation and contempt of court, remains eligible because there has been no ruling yet against him.

There are no public opinion polls in Senegal, but Faye is seen as a strong contender among the 19 candidates vying for the presidency - with the campaign condensed into a little over two weeks after uncertainty of the date of the vote was settled by a court ruling after widespread protests.

Members of Sonko's dissolved Pastef party and other parties that formed a coalition to back Faye, tax inspector, launched his campaign in his absence on Saturday.

High-profile politicians, including former prime minister Aminata Toure, who had a public falling out with outgoing President Macky Sall and left the ruling coalition last year, have thrown their support behind Faye.

Supporters with bright bracelets and campaign posters bearing the images of both Faye and Sonko have joined the campaign caravan crisscrossing the country.

"We are here for the project. Even in prison, our resolve remains strong. I left everything for this project. I'm a tailor but I left everything to help this project," said Cheikh Omar Sow, who joined a throng of supporters in Dakar on Tuesday.

The Faye coalition has promised institutional changes, and plans to create a new national currency, which could have significant implications for the eight-nation West African Economic and Monetary Union. It also plans to renegotiate mining and energy contracts.

Both Faye and Sonko were expected to be released after parliament passed an amnesty bill on March 6, aimed at easing the political tension, but it remains unclear if they will be released before election day.

Their lawyer told Reuters on Wednesday and there was no indication that they would be released any time soon.