The party of Liberian football star George Weah has warned the elections commission not to declare a winner in last week's presidential run-off.
The last votes have now been counted and Ellen Johnson-Sirleaf has 59% to 41% for Mr Weah, who claims the vote was rigged.
An official of Mr Weah's CDC party said if Mrs Johnson-Sirleaf is declared the winner, there will be "resistance".
The elections are the first since the end of 14 years of civil war.
Mr Weah has urged his supporters - who include many former combatants - to remain calm while an investigation is held into the allegations of election fraud.
This is due to start on Wednesday.
Both western and African observers have praised the election as broadly free and fair.
Announcing the last results, National Elections Commission (NEC) chairwoman Frances Johnson Morris warned Mrs Johnson-Sirleaf and her supporters not to start celebrating yet.
"Madame Sirleaf has no right to declare herself the winner. Only the NEC does," she said.
She said the results would be confirmed by 23 November after the poll complaints had been investigated.
On Monday, hundreds of activists from Mr Weah's CDC (Congress for Democratic Change) marched through the capital, Monrovia, to the US embassy, where they handed in a petition demanding that the election be nullified and a new poll held.
CDC official Max Doyen said that if the National Elections Commission announces the results "they will meet up with resistance from the Liberian people."
"We will intensify our non-violence approach; we will spread it to the 15 counties; we will ground this county; we will demonstrate every day and we will make sure the entire election result is reversed until a free and fair election is held."
The 18 CDC members elected to parliament have already threatened to boycott the legislature "if the massive electoral fraud is not addressed adequately".
The CDC won more seats than any other party.
If Mrs Johnson-Sirleaf's victory is confirmed, she would become the first woman to be elected head of state anywhere in Africa.
She rejects claims of vote-rigging as absurd.
She told the BBC she hoped Mr Weah would join her new government after "getting over his disappointment".
The election was held after the 14-civil war ended two years ago.