Senegalese opposition firebrand Ousmane Sonko and the presidential candidate he is backing in the March 24 election, Bassirou Diomaye Faye, were released from prison on Thursday, state broadcaster RTS said on its website.

Their release had been expected after parliament passed an amnesty law on March 6 as the authorities sought to ease tensions following their thwarted bid to postpone the vote by 10 months.

RTS did not give further details, and it was not immediately clear where the pair were after their release.

Thousands of supporters massed in Dakar, the capital, to celebrate the news, chanting Sonko's name on the gridlocked street where he lives. Some lit flares, danced or tooted their motorbike and car horns.

"We've been waiting for this day for so long. Prayed for it," said health worker Fatima, 52, who gave only her first name. She had rushed to join the crowd when she heard Sonko and Faye were free.

"I believe Sonko can change the country," she said.

Sonko has called on his supporters to back Faye in the presidential race, a concern for competitors as Sonko enjoys widespread support, particularly among young people frustrated with economic hardships and a lack of jobs in the West African country of 17-million.

Members of Sonko's dissolved Pastef party and other parties formed a coalition and picked Faye as a candidate in November. Sonko himself was disqualified over a defamation conviction he said was politically motivated, which authorities deny.

Faye, who previously was relatively unknown but also in detention, remains eligible because there has been no ruling as yet against him linked to charges including defamation and contempt of court.

There are no public election opinion polls in Senegal, but Faye is seen as a strong contender among the 19 candidates vying to succeed President Macky Sall, who is stepping down after two terms.

Earlier on Thursday, the head of Amnesty International Senegal, Seydi Gassama, said authorities had sought to limit Faye's electoral chances by keeping him in prison, seeing him as the main threat to the ruling coalition's candidate, Amadou Ba.

"It's really regrettable that we're at this point because ... it shows that we're not ready to organise a free and democratic election," Gassama told Reuters ahead of Sonko and Faye's release.