Sambi, 64, the arch opponent to current president Azali Assoumani, faces charges related to the alleged sale of Comorian passports to stateless people living in Gulf nations.
"He betrayed the mission entrusted to him by the Comorians," said public prosecutor Ali Mohamed Djounaid before the elite State Security Court.
He demanded life imprisonment for Sambi, who will be sentenced on November 29.
The ex-leader is accused of embezzling millions of dollars under the scheme.
But Sambi's French lawyer, Jean-Gilles Halimi said there is no evidence of this money -- "no account discovered".
Sambi refused to attend the trial, as his lawyers said there were no guarantees he would be judged fairly.
He only appeared on Monday with his lawyers asking the judge to recuse himself because he had previously sat on the panel that decided to indict him.
Sambi has already spent four years behind bars and was originally placed under house arrest for disturbing public order.
Three months later he was put under pre-trial detention for embezzlement, corruption and forgery, in the so-called "economic citizenship" scandal.
He was then charged with high treason.
Sambi, who led the small Indian Ocean archipelago between 2006 and 2011, passed a law in 2008 allowing the sale of passports at an exorbitant fee.
"They gave thugs the right to sell Comorian nationality like we would sell peanuts," said Togolese lawyer Eric Emmanuel Sossa, who is sitting on the prosecution bench.
Among the defendants was French Syrian businessman Bashar Kiwan who accused the government of seeking to pressure him into testifying against the former president in exchange for a pardon.
The presidency has formally denied these accusations but the defence has indicated to AFP its intention to file a complaint for tampering with witnesses.
The Comoros islands -- Anjouan, Grande Comore and Moheli -- have endured years of grinding poverty and political turmoil, including about 20 coups or attempted coups, since independence from France in 1975.