Zimbabwe's President Emmerson Mnangagwa led tributes to late opposition leader Morgan Tsvangirai on Thursday, describing the long-time rival of recently ousted president Robert Mugabe as a dedicated defender of democracy.

Arguably Zimbabwe's most popular politician, Tsvangirai died on Wednesday evening aged 65 after a long battle with cancer.

His death in a South African hospital cast his Movement for Democratic Change (MDC) party into unknown territory less than six months before elections, the first in Zimbabwe that will not feature 93-year-old Mugabe.

"He remained a national figure who obdurately insisted on free, fair, credible and non-violent elections as a way of strengthening our democracy," Mnangagwa, who replaced Mugabe in November after he was forced to stand down by the military, said in a statement.

In political terms, the ruling ZANU-PF party and Mnangagwa, who paid a personal visit to Tsvangirai in January, stand to gain from the opposition leader's death, due to fighting within the MDC over who should succeed its founding father.

The MDC's National Council announced the party's vice president, Nelson Chamisa, was taking the role of acting president following Tsvangirai's death.

The MDC described Tsvangirai as a hero who had "left his footprints on the sands of history", in large part through his tussles - both literal and figurative - with Mugabe.

Tsvangirai suffered repeated abuse and harassment from Zimbabwe's security and intelligence services, including a 2007 beating in police custody that left him with a badly swollen face and a deep gash in his head.

Scores of his supporters were beaten and killed during campaigning for a 2008 election that eventually resulted in four-year unity government with Tsvangirai serving as prime minister to Mugabe's president.

The tense relationship between the two was tempered by moments of respect and humour, and the pair once joked about Tsvangirai fearing Mugabe would try to poison him when they started their custom of taking afternoon tea together every Monday.

In central Harare, crowds of people wearing bright red MDC t-shirts gathered outside the party headquarters. Many wept with grief.

In a sign of respect, the government said it would help pay for the repatriation of Tsvangirai's body and his funeral.