Supporters of Liberian presidential candidate George Weah have clashed with United Nations peacekeepers after official results showed him losing.
Many Weah supporters are former gunmen from the civil war
At least one person was injured when UN forces fired tear gas and wielded batons as hundreds of people protested in Monrovia at alleged election fraud.
With 97% of ballots counted, Ellen Johnson-Sirleaf has a lead of 18% over Mr Weah, official results show.
Mr Weah has called on his supporters to react peacefully to news of his defeat.
If Ms Johnson-Sirleaf's victory is confirmed, the "Iron Lady" would be the first woman to be elected president of Liberia - or anywhere in Africa.
In another development, the UN Security Council unanimously adopted a resolution calling for peacekeepers in Liberia to detain former President Charles Taylor if he returns to the country.
Mr Taylor, now living in Nigeria, is accused of war crimes and crimes against humanity by a UN-backed court in Sierra Leone.
'No more violence'
As foreign observers declared the vote "peaceful and transparent", the 15,000-strong UN mission deployed extra troops on the streets in case of any trouble.
Mr Weah has said Ms Johnson-Sirleaf should not claim victory while his complaint to the Supreme Court is being investigated.
UN peacekeepers in riot gear used tear gas and batons to disperse the crowd near the US embassy.
They acted after some Mr Weah's supporters broke through a line of Liberian riot police trying to hold them back from the embassy, Reuters news agency reports.
The clash happened despite Mr Weah's earlier appeal to supporters "not, in the name of peace, to go on the streets".
"There is no need to cry because we have not lost the election," Mr Weah told them at the headquarters of his Congress for Democratic Change (CDC) party before they marched to the National Elections Commission.
"The streets of Monrovia do not belong to violent people. Leave the streets of Monrovia. People are frightened. They want no more war."
The Weah camp, which is supported by most of the 100,000 ex-combatants from Liberia's 14-year civil war, has alleged has alleged ballot tampering, intimidation and harassment during Tuesday's vote.
On Friday, the CDC filed a "writ of prohibition with the Supreme Court of to intervene and stop the counting process", party spokesman Steve Quoah told reporters.
The BBC's Mark Doyle in Monrovia says the aim is to get the vote counting stopped so Ms Johnson-Sirleaf cannot be officially declared president.
Ms Johnson-Sirleaf rejected claims of vote-rigging as absurd and said she was not too worried by the protests.
"Once they have got this out of their system and realise I will be a leader for all Liberians, they will settle down and we will work together for the good of our country," Mrs Johnson-Sirleaf told the BBC's World television.
She said she hoped Mr Weah would join her new government after "getting over his disappointment".
The head of the EU observer mission, Max van den Berg, has said the vote was "well administered in a peaceful, transparent and orderly manner".
Observers from the Economic Community of West African States also deemed the vote fair.
The election was held after the civil war ended two years ago.