The claims came after a violence-tinged campaign by the two leading candidates in a country usually known for relative stability.
"The ECZ (Electoral Commission of Zambia) is somehow conniving with people in the political arena to delay the release of results so that PF (Patriotic Front) thugs armed with guns take over the polling stations at night and write fake results," United Party for National Development (UPND) candidate Hakainde Hichilema told journalists.
At least three people were killed in the lead-up to the vote, with regular clashes erupting between PF and UPND activists.
The commission earlier stated that the unrest was "unprecedented" and had "marred Zambia's historic record of peaceful elections".
After a calm day of voting yesterday, early results were expected today, with the complete tally due out at the weekend.
But a scheduled briefing today was repeatedly delayed, leaving voters anxious.
Hichilema urged his supporters to stay peaceful.
"What is going on is collusion between the PF, police and the ECZ to steal this result," he said.
Fewer than 28,000 votes separated Lungu and wealthy businessman Hichilema in a snap 2015 ballot that brought Lungu to power.
Constitutional changes mean that the winner must now secure more than 50 percent of the vote, pointing to a possible second round run-off.
"The counting of votes is ongoing in polling stations countrywide," electoral commission director Priscilla Isaac said earlier at a briefing.
"The commission wishes to reiterate that all election results appearing on social platforms are unofficial (and) continues to call for calm and peace until the whole process is concluded."
But the skirmishes continued, including fighting in the streets near Hichilema's final election rally.
Hichilema has previously claimed that fraud denied him victory last year.
And this year, he charged that his campaign was marred by the authorities banning rallies, arresting party leaders and through biased state media coverage.
Disclaimer: No Business Standard Journalist was involved in creation of this content
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