More than half the votes have been counted and President Museveni has 63 per cent of the vote with his nearest challenger Kizza Besigye at only 34 per cent.
Mr Museveni’s National Resistance Movement (NRM) has already claimed victory in the ballot which would see the president take a third elected term in office, extending his 20-year hold on power with a new five-year mandate.
"We have won this election," said NRM spokesman Ofwono Opondo. "We have won both the presidential and parliamentary vote," he said.
But Mr Besigye’s Forum for Democratic Change has alleged serious irregularities in voting and the vote reporting.
“We will question the final results if we see irregularities in the way they have been announced," FDC chairman Sulaimani Kiggundu said. "We will reject them," he told reporters.
Electoral Commission chairman Badru Kiggundu denied any wrongdoing and said the claims were baseless.
A candidate needs more than 50 per cent of the vote to avoid a second run-off ballot with the next highest finisher. The FDC maintains that Mr Besigye had at least enough support to force Museveni into a second round.
Authorities estimate the voter turn-out to be at least 60 per cent of Uganda's 10.4 million eligible voters with European Union monitors commending their relatively peaceful conduct.
But the EU Election Observation Mission conceded that the ballot had been fraught with flaws and had warned of post-election violence.