Police are investigating 21 complaints in which voters said others used their identities to cast their ballots.
They include two women who arrived at a Sha Tin polling station to find they were recorded as already having voted.
"I am very angry. Why was I deprived of my right to vote?" asked one of them, Yip Man-fei, 40, a secretary who lives in Sha Tin City One.
She complained to the staff, who said she could still vote, but the word "duplicated" would be stamped on her ballot ticket.
Later another woman, 48, who went to the same polling station, was also told someone had voted with her identity. Police said the identity cards of the two victims had not been stolen or lost in recent months.
They added that all the cases would be handed over to the Independent Commission Against Corruption.
A complaint was also filed by a man, 45, who was unable to vote at a polling station at Kwai Chung Estate. But police said he had suffered from mental illness for about 10 years and might have voted and then simply forgotten about it.
"During inquiries at the scene, he failed to tell us where he had been in the past few hours," a police officer said. "We suspect he had voted but forgot it."
The officer said the chance of human error by staff at voting stations was slim because they carefully checked the names and identity card numbers of voters.
Meanwhile, police were called in when 200 supporters and candidates complained that they could not get into the counting centre at AsiaWorld-Expo because the public gallery was full at 5.30am.
Some were involved in scuffles with security guards before they were allowed in.