The southern African country heads to the polls on Aug. 23 to elect the president and legislature in what analysts expect to be a tense affair, marked by a crackdown on dissent and fears of vote-rigging.
The opposition Citizens' Coalition for Change, CCC, was campaigning in a southwestern suburb of the capital Harare on Tuesday when supporters were blocked by police, according to party spokeswoman Fadzayi Mahere.
Police confirmed they had arrested 40 CCC activists, alleging the party notified authorities it would hold a rally, but then diverted from the planned location.
The group "went on a car rally procession" in a nearby area, and stopped at a traffic light "openly blocking traffic," police said, adding that CCC supporters "started chanting party slogans and singing."
Footage shared on social media showed dozens of people clad in the CCC's yellow colors, some crammed in the back of a small truck, crowding an intersection.
The opposition has long complained about being unfairly targeted by authorities in the run-up to the election, with its members arrested and dozens of CCC events blocked.
A report by Human Rights Watch this month said the upcoming ballot will be held under a "seriously flawed electoral process" that does not meet global standards for freedom and fairness.
It accused police of "partisan conduct" and of using "intimidation and violence against the opposition."
President Emmerson Mnangagwa, 80, who heads the ZANU-PF, in power since independence in 1980, is seeking re-election in the presidential vote next week.
His main challenger is CCC leader Nelson Chamisa, a 45-year-old lawyer and pastor.