Liberia's Supreme Court has stayed next week's presidential run-off election until it considers a challenge to first round results by a losing candidate who has alleged fraud.

Third-place finisher Charles Brumskine's Liberty Party contested the results of last month's vote, which set up a November 7 run-off between former soccer star George Weah and Vice President Joseph Boakai.

The election is meant to usher in Liberia's first democratic transition since 1944, after periods of military rule and civil war that ended in 2003.

In a writ issued late on Tuesday, the court instructed the Liberty Party and the National Elections Commission to file briefs by Thursday. It was unclear if the court would rule before November 7.

"This is a big step in the right direction," Liberty Party Chairperson Benjamin Sanvee said in a statement. "Thankfully, the Court recognises the gravity of the issues, and has taken action in defence of the law and democracy."

Earlier this week, Boakai's ruling Unity Party announced it was backing the legal challenge. It accused President Ellen Johnson Sirleaf, one of its own members, of interfering in the October 10 vote by holding private meetings with election magistrates.

Johnson Sirleaf, who has governed for the last 12 years, denied the meetings were inappropriate. International observers like the European Union and the Carter Center have said that they saw no major problems with the first round vote.

The court's decision points to the increasingly assertive role some African judiciaries are playing in overseeing elections.

Kenya's Supreme Court annulled the results of a presidential election in August, although the re-run last week was marred by an opposition boycott.

In Liberia's capital Monrovia on Wednesday, riot police deployed outside the Supreme Court and election commission headquarters as authorities urged calm.

"The public should not panic," Sam Collins, a police spokesperson, told local radio. "People can go about their normal business."

Weah won the first round with 38.4 percent of the vote to Boakai's 28.8 percent and picked up an important endorsement last week from former warlord Prince Johnson, who won 8 percent of the first-round vote.

Morluba Morlu, a senior official from Weah's CDC party, said on Wednesday that he still expected the run-off to go ahead next week.

"It is sad for a ruling party that has been in power for 12 years (to) be crying," he said of Unity Party's support for the legal challenge. "We don't want any mockery of this election."