Shots have been fired during an opposition protest in Monrovia ahead of Liberia's presidential run-off.Hundreds of opposition supporters are said to have exchanged fire with the police.Congress for Democratic Change (CDC) candidate Winston Tubman has pulled out of Tuesday's vote, alleging fraud.Nobel Peace laureate Ellen Johnson-Sirleaf, Africa's first elected female head of state, won the first round but failed to pass the 50% threshold.Liberia is still recovering from its 14-year civil war, which ended in 2003.These are the first elections organised by Liberians since the conflict ended - the previous ones were run by the large UN peacekeeping mission.The rioting broke out after CDC supporters gathered outside party headquarters.Police, backed by UN forces, said they were not allowed to hold a march.