Ellen Johnson-Sirleaf has been declared the winner of Liberia's presidential poll, making her Africa's first elected female head of state.
Ms Johnson-Sirleaf is a veteran economist and politician
She took 59.4% of the vote in elections earlier this month, compared to 40.6% for former football star George Weah.
Mr Weah alleged the run-off vote was rigged, but international observers say the poll was largely free and fair.
UN peacekeepers have been deployed in the centre of the capital and traffic barred from the area to ensure peace.
The polls were the first since Liberia emerged from 15 years of civil war.
The announcement of Mrs Johnson-Sirleaf's win is a symbolically crucial day for Liberia, says the BBC's world affairs correspondent Mark Doyle.
Mrs Johnson-Sirleaf, a former World Bank economist and veteran politician, says she hopes it is also a day that will serve as an inspiration for women across Africa.
She is nicknamed The Iron Lady, but has promised to show a new, softer side as president.
The official results came some two weeks after the polls took place partly because of the logistical difficulties of collecting and checking results from remote jungle locations.
Her supporters from the Unity Party said they were planning low-key victory celebrations to avoid confrontation with Mr Weah's backers.
"We had planned jubilation throughout the country - but we want to postpone it to avoid clashes," a party spokesman told AFP.
Mr Weah's supporters, who include many fighters demobilised after the civil war, have taken to the streets several times this month to protest over alleged voting fraud.
Top officials from Mr Weah's party have said they will take their case to the Supreme Court if an electoral commission, which is still investigating, finds no evidence of fraud.
The election was organized and its security guaranteed by a UN peacekeeping force.
Our correspondent says most Liberians believe a free poll in their war-ravaged country would have been impossible without the UN presence.
The head of the UN mission says the task at hand now is reconciliation and reconstruction.
After a quarter of a century of war and misrule, Liberia's road network is in ruins, there is no national telephone network and no national electricity grid.