President Paul Biya's rival in a 2018 presidential race, Maurice Kamto, and his 90 co-accused appeared briefly before a military tribunal on charges of insurrection and rebellion after rejecting the vote.
The 65-year-old Kamto has spent months protesting the outcome of a presidential election in October 2018 that gave Biya a seventh term in office. The charges he and his co-accused are facing carry the death penalty.
Security forces were heavily deployed around the court in the capital Yaounde ahead of Kamto's arrival. Hundreds of supporters of the Movement for the Rebirth of Cameroon (MRC) leaders gathered outside. Dozens of representatives of opposition parties were in court.
The court proceedings were suspended and adjourned to 8 October 2019 after Kamto ally Penda Ekoka fainted and was taken away for medical treatment.
"All of us are asking that Kamto should be freed. He has not been installed as a president legally, but legitimately, he is the president," Ali Ibrahim, a Kamto supporter, told DW.
"Some sort of verdict may rather cause something terrible. We all know that his arrest was illegal," Fabrice Lena, the national communications secretary for the opposition Popular Action Party told DW.
"He (Kamto) should be liberated for the purpose of national reconciliation, peace and dialogue."
The trial is the culmination of a brutal crackdown on opposition politicians and dissent in the wake of the presidential election of October 2018. Kamto, who leads the Movement for the Rebirth of Cameroon (MRC) and some 200 of his supporters were arrested in January 2019 over regular demonstrations against the vote.
International pressure is mounting on Biya over the case brought against Kamto and the violent crackdown on armed separatists in the country's Anglophone regions. The European Union, colonial-era power France in particular, and the United States are among the most vocal critics of the Biya administration.
Lawyer sees 'no justification'
Read more: Analysts see a bleak future for Cameroon
"There is no justification for Mr. Kamto and his supporters to have been incarcerated for eight months in these conditions," Antoine Vey, their lawyer told the French news agency AFP.
"None of them took part in acts of violence, none called for acts of violence or rebellion; there is no reason for their arrest other than a political motive."