Liberian President Ellen Johnson-Sirleaf urged voters Saturday to ignore a poll boycott called for by her rival, saying the move was illegal and intended to intimidate Liberians.

Her calls were echoed by both the United States and West African regional body ECOWAS who criticised the move and indicated that they would recognise the vote anyway.

Winston Tubman, the challenger in a presidential run-off vote due on November 8, said Friday he would not take part in or recognise the results of the vote because the process was biased against him.

The boycott risks derailing Liberia's second post-war election, viewed as a measure of how far the minerals-rich nation has come since years of war ended in 2003.

Tubman called for a demonstration Saturday but, by late afternoon, there were no signs of notable protests. Security was heightened though, with joint United Nations and Liberian police patrols in the streets at slow speed with sirens on.

Do not succumb to fear and intimidation. Do not allow any politician to hold our country hostage, Johnson-Sirleaf said in a radio address to the nation.

Do not allow Mr Tubman to falsely claim boycott when what he is doing is forfeiting the rights of the finals because he fears defeat, she added.

The U.S. State Department said it was disappointed by the decision of Tubman's CDC party and encouraged Liberians to participate in the run-off election: The CDC's charge that the first-round election was fraudulent is unsubstantiated.

ECOWAS accused Tubman of trying to undermine the election process and going back on talks he had with it.

ECOWAS also said that so long as the November 28 vote was seen to pass off smoothly and is endorsed by credible observers, the West African group would recognise the result.

Johnson-Sirleaf, a newly-named Nobel Peace Prize laureate, led after the first round with 43.9 percent of the vote and has since secured the support of third-placed, former rebel leader Prince Johnson, who won 11.6 percent.

Tubman, a former U.N. diplomat who took 32.7 percent in the first round, has sought to have the election commission revamped in between the rounds.

The election commission chief resigned as a result but Tubman said the changes were not enough and Friday said he would never grant legitimacy to a corrupt political process.

MOOD IS QUIET

In a sign of mounting concerns over the situation, the United Nations, which still has a 9,000-strong peacekeeping force in the country, issued a statement after the boycott call Friday calling on Liberians to refrain from violence.

Tubman had urged supporters to gather at his party headquarters, but by late afternoon there were only about 500 people there and the mood was quiet.

There is no trouble. We're just hoping to be done with this. There is no competition now, said a Monrovia resident who identified himself as Edward.

Elsewhere in the capital, banners adorned the sides of buildings, most of them in support of Johnson-Sirleaf. A group of several dozen people marched down a road, blocking traffic with a banner calling for the president's re-election.

Johnson-Sirleaf has been credited with restoring stability and starting development after 14 years of conflict. But critics argue progress has been too slow and issues like corruption and poor basic services have not been addressed.

While many Liberians are tired of years of unrest, analysts say Tubman and running mate George Weah, a former football star popular with the youth, may try to tap into simmering public frustrations.

Billions of dollars in mining and oil deals have been signed since Johnson-Sirleaf came to power after elections in 2005 and a successful vote would likely intensify investment interest in the West African nation.

(Additional reporting by Ayesha Rascoe in Washington; Additional reporting and writing by David Lewis)