The Liberian election commission was upended after a series of allegations surfaced that it was quietly backing incumbent President Ellen Johnson Sirleaf.
Sirleaf won the most votes in last month's election but not the majority needed for an outright victory.
Police in Monrovia fired tear gas at protesters rallying in support of the runner-up candidate Winston Tubman.
Correspondents with Voice of America described a tense situation in the capital, where at least one person was dead ahead of a runoff vote pitting Tubman against Sirleaf.
Tubman, however, has joined calls for an election boycott Tuesday, claiming the original vote was rigged.
Liberian National Election Commission said it was investigating more than a dozen of the 54 complaints filed in connection with the October vote.
The U.S. State Department said election violence in Liberia was unacceptable, adding it had dispatched representatives to observe Tuesday's vote.
Victoria Nuland, a State Department spokeswoman, said Washington was "deeply disappointed" by Tubman's calls for an election boycott.
"Participation in elections is a fundamental part of democracy," she said in a statement.
Last month's election was the second since the end of the country's civil war in 2003 and the first ever conducted independently by the Liberians.
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