Allegations of fraud by supporters of ex-rebel leader Jean-Pierre Bemba in the Democratic Republic of Congo are unsubstantiated, say poll officials.
Electoral commission head Apollinaire Malu Malu called for candidates to respect election rules after Mr Bemba's coalition claimed he is leading.
President Joseph Kabila looks poised to win with some 90% of votes counted.
Provisional results put Mr Kabila on 58%, ahead of Mr Bemba on 42%, the Independent Electoral Commission said.
Final results were not yet available but the commission published results from all 169 voting districts on its website.
Final results must be published by Sunday.
But the Bemba camp said their candidate had received more than 50% of the vote and that victory "was being stolen from the Congolese people".
The IEC stresses that no "trend projection" can be made on the basis of the provisional results.
An analyst who wished to remain anonymous told the BBC that there were serious questions about the validity of some ballot papers, especially a large number of votes cast by voters outside their home areas.
'Logical'
Mr Malu Malu said that accusations of fraud had to be backed up with proof.
RUN-OFF RESULTS
"Elections need precision and this precision is a guarantee of transparency. Those who want to proclaim themselves are just impostors," he said.
"They fool no-one but themselves."
Bemba's coalition said in a statement, that if the CEI had cheated, they would not feel bound to comply with an earlier promise to respect the outcome of the election.
"The Union for the Nation will not accept an electoral hold-up that aims to steal victory from the Congolese people," the coalition said in a statement
The vote has been the first following DR Congo's five-year conflict.
International peacekeepers have deployed extra troops in the capital, Kinshasa, which is a Bemba stronghold and a likely flashpoint for any trouble.
Forces loyal to the two candidates clashed during the war as well as during the tense election period.
Regional divide
Following violence on Saturday in which four people died, the police arrested 337 homeless people, including 87 children, the government says, blaming them for starting the trouble.
Eyewitnesses say that security forces loyal to the two candidates exchanged gun- and mortar-fire.
United Nations observers say the election is the most significant in Africa since Nelson Mandela was elected as South Africa's president in 1994.
The first round of elections showed a regional divide, with Mr Kabila gaining a landslide in the Swahili-speaking east, while Mr Bemba got most support in the west, where Lingala is the common language.
The world's largest peacekeeping force - 17,000-strong - is in DR Congo, tasked with ensuring security.
At least 23 people were killed in gun battles between security forces loyal to the two men in Kinshasa after the announcement of first round results.
Mr Kabila won 45% of the vote, while Mr Bemba got 20%.
International observers generally praised the vote as being well-run, despite some disruptions in the north-east of the country.
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