Voting has begun in Liberia's presidential run-off despite at least one death during opposition protests and a boycott over fraud claims. Opposition candidate Winston Tubman said he was pulling out of the vote, but the election commission urged Liberians to cast their ballots. Nobel Peace laureate Ellen Johnson-Sirleaf, Africa's first elected female head of state, is seeking re-election. A BBC correspondent says her victory will be tainted unless turnout is high. She was elected in 2005, in the first election since the end of a 14-year civil war. The BBC's Jonathan Paye-Layleh in central Monrovia says far fewer people seem to be voting in the run-off, compared to the first round. At the polling station where he is, just eight people are waiting to cast their ballots, when there were hundreds last month. 'Bad signal' Officials from Mr Tubman's Congress for Democratic Change (CDC) said at least four people died after police opened fire on supporters on Monday but this has not been confirmed. Our reporter saw one body, and three or four other injured people who said they had been shot. The authorities denied using live rounds but say an investigation has been opened. These are the first elections organised by Liberians since the 14-year conflict ended. The previous ones were run by the large UN peacekeeping mission, which still has some 8,000 troops in the country. Justice Minister Christiana Tah told the BBC that security would be stepped up for the elections following the violence and that an investigation would be opened. She could not confirm the number of casualties. Mr Tubman's running mate, former football star George Weah, condemned the shooting of "unarmed protesters" and called for the elections to be postponed. President Sirleaf won the first round last month but failed to pass the 50% threshold needed for outright victory. Mr Tubman and the CDC say there was widespread vote-rigging - charges denied by the election commission and Mrs Sirleaf's supporters. The US, EU and African Union have all condemned the opposition's decision to pull out of the run-off. "It's a bad signal... political leaders must be prepared to win or lose," said former Ugandan Vice-President and head of the African Union observer mission Speciosa Wadira Kazibwe, according to the AFP news agency. Prince Johnson, a former warlord who came third in the first round, has backed Mrs Sirleaf in the run-off. While campaigning on Sunday, Mrs Sirleaf said: "I know that nobody in this country, no matter what the talk or rhetoric, nobody really wants us to go back to war."