MONROVIA  (AFP) – Liberian police and protestors exchanged gunfire Monday as a mass opposition rally on the eve of a disputed presidential election run-off turned violent.

According to an AFP correspondent on the scene, a Congress for Democratic Change supporter fired shots and riot police retaliated after responding to stone-throwing with teargas.

One woman was wounded when police struck him on the head with a baton, however it was not clear whether anyone else was injured.

Thousands of CDC supporters had gathered outside party headquarters ahead of an unauthorised march called by presidential candidate Winston Tubman, who urged voters to boycott Tuesday’s run-off pitting him against President Ellen Johnson Sirleaf.

Tensions rose when riot police blocked the road after protestors attempted to stop advancing water cannons from the United Nations peacekeeping force (UNMIL).

Two UN helicopters circled above and UNMIL troops were out in force alongside Liberia’s national police.

“You will see UNMIL staff, police and military on the streets and in the air all around the country,” said UN special representative in Liberia Ellen Margrethe Loej on UNMIL radio.

“We are here to ensure everything is peaceful and we are here to deter anyone who intends to destroy the peace.”

The protestors, who chanted slogans such as “we want justice, we want freedom”, planned a march later in the day which had not been officially approved.

“As long as they have not obtained permission from the ministry of justice the police will not allow them,” said police spokesman George Badoo.

Tubman’s call for a poll boycott has been widely condemned, sparking fears of violence eight years after a long and brutal conflict that left some 250,000 dead ended.