Gunfire rocked the Comoran capital Thursday after police arrested a defeated presidential hopeful who was named head of an opposition transitional authority hoping to unseat President Azali Assoumani.
An aide confirmed the arrest of former military colonel Soilihi Mohamed who came third in Sunday's poll which the opposition said was rigged by the government.
Civilians deserted the streets of Moroni and taxis stopped collecting passengers, an AFP correspondent saw, following the crackle of gunfire around the Kandani military base.
Police earlier broke up a women's protest march and arrested 12 demonstrators.
Comoros has had a volatile political history since independence and has endured more than 20 attempted coups, four of which were successful.
The opposition has flatly rejected Azali's victory as the angry mood threatened to spark a new political crisis on the Indian Ocean archipelago.
Mohamed had previously said that the opposition "will use all peaceful means to oust the government".
Azali led a coup, then ruled the country between 1999 and 2006, and was re-elected in 2016, left his 12 poll rivals trailing.
'We have had enough'
Both observers and community groups have questioned the credibility of the election, which saw Azali declared the winner with almost 61 percent of the vote.
Polling stations were ransacked, ballot boxes were stuffed by police and observers were prevented from overseeing the integrity of the vote, according to several witnesses.
Those arrested on Thursday were part of a march by roughly 100 women to the Supreme Court where they were planning to appeal for the results to be nullified.
Armed police dispersed the protesters, and 12 were arrested for unspecified crimes.
"We have had enough of what's happening. We wanted to deliver a memorandum to the Supreme Court but we weren't able to," march organiser Samoa Abdoulmadjid told AFP.
"The police arrived and started rounding people up."
Opposition parties have opted not to appeal the vote's outcome but called instead for new polls to be held as soon as possible.
Interior Minister Mohamed "Kiki" Daoudou has dismissed criticism of the polls' conduct and declared: "It's not the street which rules the country."
For months the opposition has accused Azali of behaving like a dictator.
Azali staged the poll after Comorans voted in a referendum, boycotted by the opposition, to support the extension of presidential mandates from one five-year term to two.
He could theoretically rule until 2029, critics say, and several opposition figures were arrested around the time of the referendum.
The change upset a fragile balance of power established in 2001 that sought to end separatist crises on Anjouan and Moheli islands, and halt an endless cycle of coups.