President Johnson Sirleaf was awarded the Nobel Peace Prize last week.

But Tubman, 70, says she has not done enough to tackle corruption.

Mrs Sirleaf won the 2005 elections after the conflict ended in 2003. She defeated former football star George Weah, who is Mr Tubman's running mate this time.

Hundreds of people started queuing to vote before dawn despite the heavy rain at the polling station in the city centre, according to the BBC.

"All the voting places have been opened at 08:00 [local time and GMT], there are no reports of delays yet and no reports of incidents but the day is still long so we are waiting to hear from our stations in other parts of the country," National Elections Commission spokesman Bobby Livingstone told the AFP news agency.

Mrs Sirleaf had said she would only seek a single term but explained her U-turn by saying she wanted to finish the work she had started.

Our correspondent says central Monrovia has been transformed since the end of the war, with roads paved and many new buildings.

While Mrs Sirleaf is well regarded by the international community, some analysts say she is less popular at home and predict a tight race, possibly going to a run-off.

Her 15 challengers accuse her of not doing enough to improve the lives of ordinary people, who remain among the poorest in the world.

"One out of every three Liberians cannot feed themselves. They live in abject poverty. And they couldn't care less about the Nobel prize," said 60-year-old opposition candidate Charles Brumskine.

Mrs Sirleaf has also been criticised for backing former President Charles Taylor - currently on trial at The Hague for alleged war crimes - when he began his rebellion in 1989.

She has apologised and the pair later fell out but Liberia's Truth and Reconciliation Commission said she should be barred from holding public office.

The campaigning was generally peaceful but some 8,000 UN peacekeepers have been deployed across the country to prevent any violence.

Polling is due to end at 18:00 GMT.

Voters are also choosing members of the House of Senate and House of Representatives.

Liberia is Africa's oldest republic - it was founded in 1847 by freed US slaves, hence its name.