Sonko had warned on Sunday night he would no longer respond to court summonses without guarantees for his safety. And when the appeal opened he was nowhere to be seen, an AFP reporter at the courthouse said.
"We have never seen in the whole world a citizen summoned to court, who turns himself in, be the subject of so much fierceness, so much brutality and an attempt almost at physical liquidation," Sonko said on social media Sunday.
He appeared to refer to his claims of a March assassination attempt by police during his forced transfer to the Dakar court during initial defamation proceedings.
"It is no longer justice, it is judicial banditry, and that is why I made the decision, still within the framework of my civil disobedience campaign, to no longer collaborate with this justice" without security guarantees, Sonko said Sunday.
Sonko came third in the 2019 presidential election and intends to stand again in 2024, but faces two court cases that could ruin his candidacy.
He received a suspended sentence and a hefty fine in March for defaming tourism minister Mame Mbaye Niang and the court was due to hear his appeal of that judgment Monday.
If the court decided to increase his sentence at the appeal, it could make him ineligible for the election.
He is also facing trial later this month for alleged "rape and death threats" over a complaint filed by an employee at a beauty salon where he went for a massage.
In addition to appealing his defamation conviction, Sonko has denied the rape charge, saying he is the victim of a plot by President Macky Sall to torpedo his 2024 candidacy.
The government has denied the accusation, and alleged Sonko has drummed up demonstrations to avoid justice.
Sonko's appointments with the justice system are often sources of tension and near-paralysis of traffic in Dakar, as he usually travels to court with a procession of sympathizers.
The rape charge against Sonko in 2021 helped trigger riots that left at least 12 people dead.