Sonko was greeted by hundreds of supporters outside a hotel in the capital Dakar, where he held a news conference with his deputy and presidential hopeful Bassirou Diomaye Faye, also released from prison late Thursday.
Sonko predicted a convincing first-round victory for Faye if the March 24 vote went ahead without fraud.
"If the election goes well, I don't think we'll get less than 60 percent," he said.
He called on "the Senegalese people to remain vigilant, especially as there are rumors of corruption circulating," while adding that he did not have "sufficient evidence" on the matter.
Disqualified from running in this vote, 49-year-old Sonko has been at the center of a more than two year stand-off with the state.
Sonko named Faye to replace him on the presidential ballot and in Friday's speech, he re-iterated his backing for him.
"My person is not the most important thing. My objective is to help win the election," he said.
"I did not choose Diomaye by chance," he added, referring to Faye.
'Support and solidarity'
Sonko reserved his sharpest attacks for the presidential camp's candidate, former prime minister Amadou Ba.
But he insisted "the election will not be a war."
"There will be no revenge, there will be justice," Sonko said, referring to accountability for those currently in power should his camp win.
Faye is thought to be one of the favorites to win the presidential election.
His coalition had kicked off the campaign while he was still behind bars, pitching him as the candidate for "system change" and "left-wing pan-Africanism."
The opposition figure has promised to reclaim Senegal's sovereignty and renegotiate oil and gas contracts, as well as defence agreements if elected.
In the early hours of Friday, thousands thronged the streets of Dakar, singing and dancing as Sonko and Faye were driven away from the city's Cap Manuel prison.
Upon his release, 43-year-old Faye, smiling and raising his arms to the sky after almost a year behind bars, praised the "support and solidarity" of his followers.
Faye and Sonko benefited from an amnesty law passed by MPs last week.
Source: Agency France Press
