Police in Comoros arrested defeated presidential hopeful Soilihi Mohamed on Thursday, a campaign official told AFP, as officers also broke up a women's protest march arresting 12 demonstrators.
Campaign official Kamal Eddin Sindou confirmed the arrest of Mohamed, who was named on Wednesday as the head of an opposition coalition to challenge President Azali Assoumani's re-election following Sunday polls.
A relative also told AFP that Mohamed had been detained.
Mohamed placed third with 3.84 percent of the vote.
The opposition has flatly rejected Azali's victory with 60.77 percent as the angry mood threatened to spark a new political crisis on the archipelago with a long history of coups and attempted power grabs.
Mohamed had previously said that the opposition "will use all peaceful means to oust the government".
Azali, who 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 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 them, and 12 of the protesters 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 in the Indian Ocean country 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 said that "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.