From Reuters:

By Richard Valdmanis and Alphonso Toweh MONROVIA (Reuters) –
Clashes and sporadic gunfire rocked part of Monrovia on Monday, killing at least one person after Liberian riot police fired tear gas to disperse several hundred supporters of presidential challenger Winston Tubman.
Members of Tubman’s CDC party said at least three other people were killed, though this could not be confirmed. Two United Nations helicopters flew overhead as police and Tubman’s rock-throwing supporters clashed in side streets.
Liberian police firing tear gas and live rounds later stormed the CDC headquarters before they were repelled by U.N. peacekeepers, who have set up a cordon around the building.
Tension has risen in the Liberian capital ahead of a November 8 election run-off between Tubman and President Ellen Johnson-Sirleaf after Tubman called on his supporters to boycott the vote over alleged irregularities, despite international pressure on him to stand.
Violence erupted after police tried to break up a crowd of several hundred CDC supporters. Shooting then broke out and a police officer said both the police and Tubman’s supporters had fired, but it was not possible to confirm the information.
…
Johnson-Sirleaf took nearly 44 percent of the first round vote on October 11 and has since won the backing of the third-place finisher, former warlord Prince Johnson, all but sealing her victory in the second round run-off.
Former U.N. diplomat Winston Tubman – who took roughly 33 percent in the first round – announced last week he would withdraw from Tuesday’s race and called on Liberians to boycott the poll due to evidence of fraud.
But international election observers called the October 11 vote mostly free and fair, and the United States, regional bloc ECOWAS and the African Union have all criticized Tubman’s decision to boycott the second round.

And here‘s an interesting commentary from Clair MacDougall at African Arguments (extract):

Opposition leader Winston Tubman’s decision last week to boycott the runoff, has also suggested that Sirleaf is the frontrunner. However, if she wins, her government could be perceived as lacking legitimacy, posing further challenges for Africa’s first female president. But with an opposition that lacks coherence and political parties without clear policy platforms or core principles and ideals, will Johnson Sirleaf’s victory further enhance or inhibit the development of a multiparty system in the fledgling West African democracy?