Thousands of opposition supporters have marched in Zimbabwe's capital to demand electoral reforms ahead of the July 30 vote, the first since Robert Mugabe stepped down last year.

Tuesday's march is Harare's largest street demonstration since the massive one in November leading to Mugabe's departure. Speakers warned they would demonstrate again if President Emmerson Mnangagwa, who took power with the military's help, "steals" the election.

Mnangagwa has promised a fair vote. But the opposition MDC-T has several demands, including an end to alleged military interference in the electoral commission, which the commission denies.

Opposition leader Nelson Chamisa said they are "prepared to do whatever is necessary" to stop elections from going ahead if demands are not met.

Police kept watch after banning the ruling ZANU-PF party from holding a counter-march.