An upcoming nationwide ballot in Zimbabwe will be held under a "seriously flawed electoral process" that does not meet global standards for freedom and fairness, Human Rights Watch (HRW) said on Thursday.

The southern African country heads to the polls on August 23 to elect the president and legislature in what analysts expect to be a tense affair, marked by a crackdown and fears of rigging.

"Zimbabwe's authorities have yet again demonstrated a lack of respect for the basic freedoms necessary for a credible, free, and fair election," said HRW's senior Africa researcher, Idriss Ali Nassah.

The US-based rights group said the authorities had adopted repressive laws to muffle dissent and used intimidation and violence against the opposition.

The courts have been "weaponised" to target opposition politicians, while the election overseers lack impartiality, the group said in a report based on interviews with activists and politicians.

It accused supporters of the ruling ZANU-PF party of having engaged in widespread harassment, threats, and violence, preventing opposition parties from campaigning.

In one incident on Thursday, the leading opposition party said one of its members was stoned to death in an ambush on the way to a rally in a suburb of Harare, the capital.

Police confirmed a person died as a result of "clashes", without immediately providing further details.

"The Zimbabwe government needs to take concrete measures before the election to meet its obligations under national and international law to allow people to vote free of intimidation, fear, and violence," Nassah said.

"So far nothing indicates the authorities are willing to do that."

Zimbabwe's government did not immediately reply to a request for comment.

President Emmerson Mnangagwa, 80, who heads the ZANU-PF in power since independence in 1980 and forced out late ruler Robert Mugabe in 2017, is seeking re-election.

His main challenger is Nelson Chamisa, a 45-year-old lawyer and pastor.