This year's presidential and parliamentary vote is a rematch of the 2008 election -- pitting 89-year-old Robert Mugabe's Zanu-PF party, which has ruled Zimbabwe since it gained independence from Britain in 1980, against Prime Minister Morgan Tsvangirai's Movement for Democratic Change, the Financial Times reported.
Tsvangirai is making his third run at the presidency.
He won the first round of voting in 2008, but withdrew from a runoff, citing a violent crackdown against his supporters. He and his party then joined Mugabe and Zanu-PF in a unity government in 2009, with Mugabe as president and Tsvangirai as prime minister.
Voters told the Times campaigning in advance of this year's voting has been relatively peaceful, with less intimidation and harassment than in previous elections.
The MDC and other organizations, however, complained of electoral irregularities, particularly with voter registration, the Times said.
Mugabe dismissed questions about registration irregularities during a Monday news conference and said he would step aside if he lost.
"If you lose you must surrender to those who won," he said.
A recent Amnesty International report accused Zimbabwean police of arresting and "intimidating human rights defenders," NBC News reported.
"The clampdown on the work of human rights defenders is a worrying indicator that government agencies remain actively hostile to civil society," Noel Kututwa, Amnesty International's Africa deputy program director, said in the report.
U.S. officials said they were "deeply concerned" about the lack of transparency in preparations for the election. Patrick Ventrell, a U.S. State Department spokesman, said the election was not considered credible.
Mugabe called the U.S. concerns "absolutely insane."
He said he has "no regrets" about his leadership.