Mozambique's main opposition leader, Venancio Mondlane, announced that he would install himself as president on January 15 after refusing to be defeated in the presidential election.
His announcement came as his supporters staged violent protests across the country to demand an end to Frelimo's 49-year rule.
The capital, Maputo, looked like a ghost town on Christmas Eve, with almost all businesses closed and people staying at home to avoid the worst unrest the city has seen since Frelimo came to power at independence in 1975.
Frelimo offices, police stations, banks and factories have been looted, vandalized and set on fire across the country.
The latest unrest began on Monday after Mozambique's highest court upheld the victory of Frelimo's presidential candidate Daniel Chapo in elections held in October.
Mundelein had contested the result, claiming the poll was rigged.
In a Facebook live broadcast to his supporters on Tuesday, Mondlane said he rejected the Constitutional Court ruling, adding that he would take over as president on January 15, the day Chapo is scheduled to be sworn in.
President Filipe Nyusi is scheduled to step down at the end of his two terms.
It is unclear how Mondelein intends to take office, as he is currently in self-imposed exile in an unknown country.
He often rallied his supporters through direct speeches on Facebook, but urged them to remain peaceful.
“We are with the people,” Mondelein said in his final speech. “We do not call for any form of violence.”
Chapo has not yet commented on his announcement.
The October elections were the first time that both of them ran for the presidency, as the Electoral Commission declared Chapo the winner with 71% of the votes compared to 20% for Mondlane.
The Constitutional Court modified the result, giving Frelimo's candidate 65% and Mondlane 24%.
Human rights groups say more than 100 people have been killed in the unrest since the election.
They accuse security forces of being responsible for many of the killings, but police chief Bernadino Rafael previously told the BBC that his officers were defending themselves after being attacked.
Mondlane fled Mozambique after accusing police of threatening behaviour, and two of his aides were shot dead in October.
The 50-year-old evangelical pastor ran in the election as an independent after separating from the main opposition party, Renamo.
Its support is strongest among young people, many of whom are unemployed and demanding change.
Frelimo's party has fielded the 47-year-old Chapo as its youngest ever presidential candidate.
He had previously rejected accusations that he and Frelimo rigged the elections, saying: “We are an organized party that prepares for its victories.”
