Violence broke out at opposition headquarters, killing at least one person hours before Liberia's presidential runoff on Tuesday, a vote that tests the West African nation's fragile peace after a devastating civil war.

Despite sharp criticism from the United States, the U.N. and election monitors, opposition leader Winston Tubman kept urging supporters to boycott Tuesday's runoff.

Demonstrators clashed with police in one rally backing the boycott, leaving one young man dead inside the headquarters of the opposition Congress for Democratic Change, or CDC, party. Nearby, four others were screaming in pain from what appeared to be bullet wounds in their legs.

Walking between the wounded, Tubman and running mate George Weah said the violence was further proof the runoff should not go ahead.

Tubman is trailing in the polls by a more than 10-point margin and the boycott is seen by many as an effort to tarnish Tuesday's election in the face of his likely defeat. The move will not stop incumbent Ellen Johnson Sirleaf, this year's Nobel Peace Prize laureate, from winning, but it could undercut her victory.

Worse, it would also cast doubt on an election that was supposed to solidify the nation's peace, just eight years after Liberia emerged from a horrific 14-year civil war that left its rolling hills and towering forests dotted with mass graves.

"This decision is unfortunate for the electoral process in Liberia, and for Liberia's young democracy," said Gilles Yabi, the director of the International Crisis Group West Africa. "It's motivated by the fact that they (Tubman's party) think they don't have a chance. It's a way to stain the election, to create a problem of credibility for the president."

The 73-year-old Sirleaf made history in 2005 when she became Africa's first elected female president and again last month when she won the Nobel Peace Prize for her role in stabilizing the country after a 2003 ceasefire.

The Harvard-trained economist is credited with luring hundreds of millions of donor dollars to her destroyed nation and getting $5 billion of its external debt wiped clean. Her critics, however, note that two out of every three Liberians still live in dire poverty and the country remains one of the least developed on the planet, according to World Bank and U.N. statistics.

Corruption and cronyism continue to erode institutions, and Tubman and Weah have complained that the country's electoral process was stacked in Sirleaf's favor.

The opposition party began threatening a boycott after the first round of voting on Oct. 11 showed that Sirleaf led with around 40 percent to the CDC's roughly 30 percent. When the third-place finisher announced he was endorsing Sirleaf, her victory seemed assured.

To participate in the Nov. 8 runoff, the CDC's demanded that the head of the election commission be replaced _ and he was.

Then last week, Tubman said the changes did not go far enough and called for the election to be postponed. Then on Friday he called for a boycott when the government refused further concessions.

Outside observers said there was no reason for the boycott.

"Liberia has taken important steps to consolidate its democracy since the end of its civil war. Those gains must not be setback by individuals who seek to disrupt the political process," U.S. President Barack Obama said in a statement. "The international community will hold accountable those who choose to obstruct the democratic process. We encourage all security forces in Liberia to exercise maximum restraint and to allow peaceful protest."

The head of the Carter Center's observation mission in Liberia, Alexander Bick, said his staff had traveled to all 15 counties in the Tennessee-sized nation and while small irregularities were noted, there was no evidence of systematic fraud.

Electoral law allows candidates to pull out before the start of the election, but once the election is already in progress, ballots cannot be altered, he said. So both Tubman and Sirleaf will appear on Tuesday's ballot. The boycott will not result in the vote being canceled.

"He is serious about wanting to boycott the election ... (but) it does not nullify the election," Bick said. "The key issue is that voters should make their choice. Some may participate. Some may not. But it should be left to the Liberian people, not to the politicians."

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Callimachi reported from Dakar, Senegal. Associated Press photographer Rebecca Blackwell contributed to this report.