KINSHASA, Congo - One of the main observation missions in Congo says the results of the disputed presidential election lack credibility.

The Atlanta-based Carter Center founded by former U.S. President Jimmy Carter had earlier said last month's vote was marred by logistical shortcomings, but that there was not evidence of systematic fraud.

In a statement released late Saturday, the Carter Center observers showed evidence of possible vote-tampering in Katanga province, a region favourable to the incumbent Joseph Kabila.

They say several constituencies reported "impossibly high rates of 99 to 100 per cent voter turnout with all, or nearly all, votes going to incumbent President Joseph Kabila."

Opposition leader Etienne Tshisekedi has rejected the results and proclaimed himself president.