Malian opposition candidate Soumaila Cisse said he would reject the results of a presidential run-off marred by accusations of fraud, violence and low turnout, calling on the population "to rise up".
Ballot counting continued on Tuesday across the vast west African country after Sunday's vote saw one poll worker killed and hundreds of stations closed due to insecurity.
"The fraud is proven, this is why there are results we will not accept," Mr Cisse said at his party's headquarters in Bamako.
"I call on all Malians to rise up ... We will not accept the dictatorship of fraud," he said.
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Malians vote in presidential run-off amid attacks and threats
Growing militancy threatens Mali's July 29 elections
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President Ibrahim Boubacar Keita, 73, is the clear front runner in a reprise of his 2013 face-off against former finance minister Cisse, 68.
But Mr Cisse's team and other opposition contenders have repeatedly accused the government of fraud, including ballot-box stuffing and vote buying.
However the African Union election observers said the voting was carried out "in acceptable conditions," in a preliminary report published on Monday.
At this stage there is "no tangible element" pointing towards voting irregularities, the observers said, congratulating the Mali government for its efforts to improve the voting process and noting a drop in the number of untoward incidents in the second round of voting.
The European Union also said that in the 300 polling stations its observers visited, no "major incident" occurred. Their observers are expected to present their preliminary findings at 11.00am GMT.
Nearly 500 polling stations were unable to open on Sunday, the government said, mostly in regions plagued by jihadist violence and ethnic tensions.
"We had a little over 3.7 per cent of stations which had not functioned properly" during the first round on July 29, Salif Traore, Mali's security minister, said on Monday.
The figure fell to 2.1 per cent of the 23,000 polling booths in Sunday's run-off vote, which Minister Traore said was due to the deployment of more military.
In a reminder of the jihadist threat that was a major campaign issue, the overseer of a polling station in Arkodia, in the northern region of Timbuktu, was shot dead on Sunday by armed militants, local officials said.
Aside from this "dramatic case," the government said the poll occurred without incident.
National turnout however was just 22.38 per cent, local monitors of the the Mali Citizen Observation Pool said.
