Voting wrapped up in Liberia's constitutional referendum Tuesday night, after a day marred by an error on the ballot paper, reports of low turnout, and some opposition leaders refusing to vote.

"Immediately at six o'clock (1800 GMT) all polls closed except the cases where some people are still in queues but we don't have any reports of such," said National Electoral Commission spokesman Bobby Livingstone.

United Nations officials could be seen carrying secured ballot boxes to the election center in the capital Monrovia, where counting would start immediately.

Two thirds of voters need to have voted 'yes' to the four proposed constitutional changes for them to be ratified. However, election officials reported very poor turnout among some 1.8 million registered voters.

Opposition parties have criticised the referendum as it could change the election date and candidacy requirements in the midst of campaigning for presidential and legislative elections on October 11.

The main opposition Congress for Democratic Change candidate Winston Tubman boycotted the referendum, and on Tuesday morning candidate Reverend Kennedy Sandy refused to vote as an error emerged on ballot papers.