Liberians lined up outside polling stations on Tuesday to vote as incumbent president Ellen Johnson Sirleaf sought a second term only days after jointly winning the Nobel Peace Prize.

United Nations peacekeepers and police deployed in numbers around polling stations in the capital Monrovia and elsewhere as hundreds queued up under light rain to vote in presidential, senatorial and legislative elections.

Around 1.8 million people are eligible to vote in only the second election since the end of a 14-year civil war and seen as a litmus test of the west African nation's fragile democracy.

"All the voting places have been opened at eight o'clock, 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 said.

John Plato, 60, was the first person to vote at the GW Gilbson High School in Monrovia, after waiting for four hours at the head of a line that stretched to 800 people.

"I feel very happy having cast my ballot. These elections are crucial in the eyes of Liberians and the international community and so it was important for me to vote to be part of the history-making process of the country."

The vote is the first since Sirleaf became Africa's first woman president in 2005, shortly after the country emerged from a savage war which left some 250,000 people dead.

She faced a stiff challenge from main opposition candidate Winston Tubman, 70, who has crowd-pleasing football star George Weah as his running mate.

Eva Anderson, a 19-year-old student, voted for the first time after waiting in line since 4:00 am.

"I feel magnificent to have demonstrated my patriotism to Mama Liberia ... as I leave here I am leaving with a clear conscience of mind that I voted for social, economic and political transformation for Liberia."

Eight years after the end of a war fought by numerous rebel factions, some of whom armed drugged-up child soldiers, the country is rebuilding slowly.

Sirleaf's government has brought electricity and running water to some parts of the capital, and rebuilt roads. However, most of the country is still extremely poor and has no access to even the most basic services.

Opposition parties say much of the population of four million has not seen change in their lives, while Sirleaf was out charming investors and getting creditors to write off billions of dollars in debt. There is an estimated 80 percent unemployment rate.

With security institutions still weak, the country is heavily reliant on an 8,000-strong UN peacekeeping force (UNMIL) and international organisations.

Observers warn the country's peace is still fragile as victims of the war seek reconciliation in a country with a dizzying array of ethnic rivalries which at times turns deadly.

Sirleaf has been criticised for dragging her feet in implementing recommendations by a Truth and Reconciliation Commission which names her on a list of people who should be barred from public office for backing warlord-turned-president Charles Taylor.

The 72-year-old grandmother on Friday became a joint winner of the Nobel Peace Prize with two other women for her work in social development and women's rights, which outraged opposition leaders for its poor timing ahead of the key election.

Voting was to close at 6:00pm and counting was to begin immediately.