Ghanaian police fired water cannon late Wednesday on ruling party supporters who besieged the country's Electoral Commission in protest at presidential election results, witnesses and local media said.

Dozens of angry protesters wielding machetes and sticks attacked passing vehicles and local journalists after partial results from the run-off vote gave the opposition candidate a lead.

Completed official results for 229 of the 230 constituencies have shown opposition leader John Atta-Mills maintaining a slim lead over governing party candidate Nana Akufo-Addo.

The demonstrators only dispersed after the regional minister for greater Accra addressed the crowd and assured them of victory after the decisive vote in a remote constituency which has emerged as decisive.

Voters in Tain, a western constituency of just over 50,000 eligible voters, will cast their run-off ballots on Friday.

They failed to vote Sunday because of problems in the distribution of ballot papers -- and because the official results so far are too close to call, the result there could effectively decide who runs Ghana for the next four years.

The protest lasted roughly two hours, according to security sources at the scene.

An AFP reporter saw broken glass and other debris in the aftermath of the demonstration. Electoral commission officials at the scene refused to comment.

Meantime, the NPP has officially lodged its complaint to the electoral commission over alleged electoral irregularities in the opposition stronghold of the Volta region.

Senior NPP officials led by campaign director Jake Obetsebi-Lamptey, were seen entering the EC offices to hand over the petition early evening on Wednesday.