Liberians head to the polls on Tuesday to elect a new president in a contest set to complete the country's first democratic transition of power in more than 70 years.

After a campaign hailed for a vibrant and violence-free debate, the small West African nation's 2.18 million registered voters will begin casting their ballots between 8am (0800 GMT) and 6pm (1800 GMT).

With Africa's first female elected head of state, Ellen Johnson Sirleaf, stepping aside after a maximum two terms, Liberians will choose from a crowded field of 20 candidates, although just one of them is a woman.

"The future of the country is in your hands, no one is entitled to your vote, not because of party, ethnicity, religion or tribal affiliation," Sirleaf, a co-winner of the 2011 Nobel Peace Prize, declared in a speech on Monday.

Among the frontrunners are footballing icon George Weah, incumbent Vice President Joseph Boakai, longtime opposition figure Charles Brumskine and former Coca-Cola executive Alexander Cummings.

Also waiting in the wings with potentially significant vote share are telecoms tycoon Benoni Urey and former central bank governor Mills Jones. 

The first official results are expected within 48 hours. If no candidate wins 50 percent of the vote, then a run-off of the top two contenders will be held on November 7 -- an outcome analysts say is a near certainty.

"There is going to be a run-off, and that is most likely to be the parties that have gone to a run-off in the last two elections," Ibrahim Al-Bakri Nyei, a Liberian political analyst at London's School of Oriental and African Studies (SOAS), told AFP.

Sirleaf's Unity Party swept the vote in 2005 and 2011, results that Weah's Congress for Democratic Change (CDC) contested in court.

"The third place candidate is also very important in this round of elections because you somehow become the kingmaker," Al-Bakri Nyei noted, placing his bet on Cummings slick, upstart campaign to cause damage to the two main groupings.

- Time for the 'chosen one'? -

Back-to-back civil wars (1989-2003) and the Ebola crisis (2014-16) have stunted growth and left Liberia among the world's poorest nations, while entrenched corruption has not been rooted out by the Sirleaf administration.

Dash Gamu, a teenage motorcycle taxi rider, said he would be voting for Weah but was not familiar with his vice-presidential pick Jewel Howard-Taylor, the ex-wife of Liberian warlord Charles Taylor.

"He is the chosen one for this nation," he told AFP.

Weah consistently captured the youth vote when he ran for president in 2005 and vice-president in 2011, but Cummings has made inroads into his support.

A fifth of Liberia's registered voters are aged 18-22 and are less likely, analysts say, to vote along ethnic lines or to support candidates like Prince Johnson, a former rebel leader who maintains a strong following in northeastern Nimba county.

- Pre-emptive victories -

Regardless of the result of Tuesday's election, the international community is keen to see Liberia's history of coups, assassinations and exiled dictators shift to a more stable footing after 12 years of peace under Sirleaf.

The election has been largely peaceful and the National Elections Commission expects the same on voting day, NEC spokesman Henry Flomo told AFP.

Liberia's police and army are overseeing election security for the first time since the civil war following a handover from UN peacekeepers last year, and though underfunded have kept the peace during the campaign.

Electoral observers from regional body ECOWAS, the African Union, the European Union and the United States will all oversee the process.

One lingering concern is the pre-emptive victory declarations made by some parties, with Weah's team calling his final rally a "victory march" and Cummings proclaiming there would be no second round.

"We all must respect the outcome of the election as declared by the National Elections Commission," Sirleaf warned in her speech on  Monday.