Zambian opposition leader Hakainde Hichilema cast his vote at a polling station in Lusaka on Thursday.
Hichilema of the United Party for National Development (UPND) is incumbent President Edgar Lungu's main challenger.
His campaign has ridden on public frustration over Lungu's economic policies which have been criticized for exposing the copper producer to high debt and soaring food prices.
After casting the vote, Hichilema told reporters that polling workers were being slow with people stranded in long queues.
"I'm just concerned about the slowness of the process. There are thousands of people, they need to vote," he said.
Hichilema is seeking the presidency for the sixth time.
Lungu voted shortly after polls opened at 6am (0400 GMT).
"We are winning, otherwise I wouldn't have been in the race if we were not winning, we are a winning team," the upbeat Lungu told reporters outside a nursery school in Chawama, a poor neighbourhood of Lusaka.