The Zimbabwe Electoral Commission says it has referred to police at least two candidates who might have violated the law by campaigning after the cutoff time.
The chair of the commission is refusing to name names during a press conference but the candidates are likely President Emmerson Mnangagwa and main opposition challenger Nelson Chamisa. Both issued public statements on Sunday.
Campaigning ended 24 hours before polls opened Monday morning.
Violations could be turned into a criminal or civil case.
Zimbabwe faces a historic, and so far peaceful, election as it seeks to move beyond longtime leader Robert Mugabe, who stepped down in November under military pressure.
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A court in South Africa has set aside a decision to grant diplomatic immunity to Zimbabwe's former first lady Grace Mugabe in a case where she was accused of beating a young model with an electrical cord.
South African media report that the South Gauteng High Court says the government decision last year was inconsistent with the constitution.
Mugabe was first lady when a young South African model accused her of assault in a Johannesburg hotel. Mugabe was allowed to leave the country, a decision that caused an outcry.
Her husband, Robert Mugabe, was forced out of office in November under military pressure after a ruling party feud as the first lady appeared to be positioning herself for the presidency.