Sporadic gunfire rocked the centre of Monrovia on Monday after Liberian riot police fired tear gas to disperse several hundred supporters of presidential challenger Winston Tubman, and at least one person was killed, a Reuters witness said.

A Reuters reporter saw a dead body at Tubman's CDC party headquarters and several people were injured, including two police officers. CDC party members said four other people had been killed but this could not be confirmed.

Tension has risen in Monrovia ahead of an election run-off between Tubman and President Ellen Johnson-Sirleaf with Tubman calling on his supporters to boycott the vote over alleged irregularities.

Several hundred Tubman supporters had faced off against riot police before the violence erupted when tear gas was used.

Two United Nations helicopters flew overhead as police and Tubman's rock-throwing supporters clashed in side streets. A U.N. vehicle was attacked.