Malawi opposition leader Lazarus Chakwera on Wednesday warned against attempts to rig the country's election, claiming he was leading as votes were slowly tallied.
Chakwera said his Malawi Congress Party (MCP) was conducting its own count, even though local observers earlier declared the election largely free and fair.
"So far the message is clear, we know that we have a tremendous lead," he told an impromptu news conference at his house in Blantyre.
"No one is going to rig this election. Justice is going to prevail."
Early official results had shown President Peter Mutharika in the lead but the electoral commission website said only 20 000 ballots had been counted.
The country has around 6.8 million potential voters but turn-out has not yet been published.
Mutharika, in office since 2014, has faced accusations of corruption and cronyism.
"Those in power, I know you, you're trying to tamper with elections," Chakwera said, who came a narrow second in the 2014 election.
"I warn you, you will soon face the long arm of the law".
Chakwera has campaigned on an anti-graft platform and has been credited with reviving the MCP.
Two months ago, he secured the high-profile support of former president Joyce Banda.
The MCP ruled Malawi from 1964 to 1994 under Hastings Banda's one-party rule but has since been in opposition.
The other main candidate contesting the election is Mutharika's own deputy president Saulos Chilima.
Nandin Patel, political science lecturer at the Catholic University in Malawi, told AFP that the election count could be "very contentious".
Malawi has a "winner takes all" system and in 2014 Mutharika won with just 36% of the vote.
He came to power in the aid-dependent country vowing to tackle corruption after the "Cashgate" scandal erupted a year earlier, revealing massive looting from state coffers.
But his government has been dogged by several high-profile cases of corruption and nepotism.
Jane Ansah, chairwoman of the Malawi Electoral Commission, told reporters that transmission problems had slowed the vote count.
The National Initiative for Civic Education, which deployed more than 5 000 monitors, said in a statement that despite isolated incidents of scuffles and disputes, election day was largely peaceful.
Foreign observer missions are expected to give their verdicts on Thursday.