Supporters of Senegal opposition leader Ousmane Sonko threw rocks at riot police and set fire to buses on Thursday after a court sentenced him in absentia to two years in jail, casting serious doubt on his chances of running for president next year.

Sonko, 48, did not attend the hearing, but Justice Minister Ismaila Madior Fall said the opposition leader could now be taken to prison at any time, and police had been stationed around his home in the capital Dakar on public safety grounds.

Sonko was accused of raping a woman who worked in a massage parlor in 2021 when she was 20 and making death threats against her. He denies wrongdoing and says the charges are politically motivated.

A criminal court cleared Sonko of rape but found him guilty of a separate offense described in the penal code as immoral behavior towards individuals younger than 21.

"With this sentence, Sonko cannot be a candidate," said one of his lawyers, Bamba Ciss, citing Senegal's electoral law.

Sonko's PASTEF party said the verdict was part of a political plot and called on citizens in a statement to "stop all activity and take to the streets."

Thick black smoke billowed from a central university campus in Dakar, where protesters set several buses alight in the afternoon after the verdict and threw rocks at riot police who responded by firing tear gas.

Government spokesperson Abdou Karim Fofana said security forces had the situation under control in the capital, although there were signs of continued pockets of unrest into the evening.

Several social media and messaging platforms were restricted in Senegal late on Thursday - a move "likely to significantly impact the public's ability to communicate," the Netblocks internet observatory said. Similar shutdowns occurred during widespread protests in 2021, it said.

University law professor Ndiack Fall said Sonko could demand a retrial if he turns himself in to authorities.

The case has triggered violent protests in the West African country since 2021. Sonko's supporters denounce the charges as a ploy to prevent him from running in elections scheduled for February. The government and the justice system deny this.