Colonel Ibrahim Salim was arrested on Monday afternoon by two military police officers "who presented him with a summons", one of his relatives told AFP, speaking on condition of anonymity.

"He came back, got changed and left again. Only then did we learn that he was in prison" in the capital Moroni, he told AFP.

It was only then that the family heard he had been imprisoned for attempted "conspiracy" and an "act of terror" against the regime, the relative said.

The arrest comes as the authorities pressed a crackdown against senior political and military figures who opposed a series of sweeping constitutional reforms which were passed during a controversial referendum in late July.

The reforms notably allow Assoumani to extend his term in office and overturn constitutional measures put in place to ensure political balance in this coup-prone country.

Like other senior figures within government and the opposition, Salim had spoken out against the reforms.

His arrest comes just days after a court formally charged Jaffar Ahmed Said Hassani -- until recently one of the Indian Ocean archipelago's three vice presidents -- with conspiring against the state.

But a day before the hearing, Hassani -- who had denounced the reforms as likely to plunge Comoros into a deep political crisis -- had flown abroad, with the court charging him in absentia and issuing an international warrant for his arrest.

Last month, state prosecutor Mohamed Abdoua announced the arrest of five people, including Hassani's brother and the writer Said Ahmed Said Tourqui.

Assoumani was elected in 2016 alongside three vice presidents, but until the referendum, he was not constitutionally permitted to dismiss them.

Following the vote, which was boycotted by the opposition, the authorities in Moroni began a crackdown, rounding up opposition figures and charging them with plotting attacks on government figures. Others have gone into hiding.

Assoumani, who had been due to step down in 2021, is now expected to stage early elections next year to extend his time in office.