Former football player George Weah's campaign said on Wednesday that he was on course to win Liberia's presidential election run-off in the country's first democratic transfer of power in more than seven decades.

Nearly 2.2 million voters are choosing between the 51-year-old Mr Weah and 73-year-old Joseph Boakai, the current vice president.

The winner will replace Ellen Johnson Sirleaf, who is stepping down after two terms.

Mr Boakai's camp said the contest was still too close to call ahead of the first official results, expected as early as Thursday.

Unofficial partial results announced on local radio stations all showed Mr Weah in the lead.

Mr Weah, the only African ever to be named Fifa World Player of the Year, lost to Ms Johnson Sirleaf in a 2005 election as a political novice after a football career that took him to both Chelsea and Manchester City. He has served in Liberia's senate since 2015.

A senior Weah adviser, Morluba Morlu, told Reuters he expected Mr Weah to win with about 70 per cent of the vote based on precinct-level vote tallies he said were trickling in from across the country after Tuesday's vote.