Former Finance Minister Ellen Johnson-Sirleaf looked set to become Liberia's next president yesterday, consolidating an apparently unbeatable lead with more than 99 per cent of the vote counted from last week's heated runoff.

Her rival, soccer star George Weah, alleged massive fraud and is contesting the ballot, though international observers have declared the vote fair.

Monrovia was quiet yesterday after an isolated clash a day earlier that saw UN forces repel rock-throwing mobs of Weah supporters with tear gas and batons outside the US Embassy.

Weah's campaign has criticised the international community for condoning the vote.

Elections Commission chairwoman Frances Johnson-Morris said 99.3 per cent of ballots had been counted so far, giving Johnson-Sirleaf 59.6 per cent, compared to Weah's 40.4 per cent.

A final tally will be announced on Tuesday, but a victor will not be declared until a complaint filed by Weah's campaign is heard on Wednesday and any other complaints are investigated, Johnson-Morris said.

The government issued a statement condemning Friday's violence.

The 15,000-strong UN peacekeeping force said it would "deal firmly and decisively with any attempts, by any persons, who would seek to use violence to derail the elections process or to undermine peace or public order."

International observers said the vote was fair and said they saw no widespread irregularities.