Zimbabwe's main opposition leader has said his party is "ready to form the next government" following the country's first general election since Robert Mugabe was removed from power.

Nelson Chamisa, 40, and 75-year-old president Emmerson Mnangagwa were the main contenders in Monday's election, which several observer groups have said is too close to call.

Mr Mnangagwa was viewed as the frontrunner, although the latest opinion polls showed a tight race. There will be a runoff on 8 September if no candidate wins more than half the votes.

"Winning resoundingly... We now have results from the majority of the over 10 000 polling stations," Mr Chamisa wrote on Twitter early on Tuesday morning. "We’ve done exceedingly well. Awaiting ZEC to perform their constitutional duty to officially announce the people’s election results and we are ready to form the next gvt."

Mr Mnangagwa later tweeted he was "delighted" by high voter turnout and that he had received "extremely positive" information from Zanu-PF representatives on the ground.

The Zimbabwe Electoral Commission (ZEC), which said on Monday voter turnout had averaged 75 per cent, will announce official results within five days.

The election came after an extraordinary intervention my Mr Mugabe, who turned against Mr Mnangagwa, his former vice president, and the Zanu-PF party he led for so long.

In his first public appearance since being removed from power, he praised Mr Chamisa, who leads the Movement for Democratic Change (MDC), as the only candidate who could “return legitimate government to the country”.

The former anti-colonial fighter went on to decry the “evil and malicious characters” who deposed him, and stridently defended his wife Grace. Ms Mugabe, who was by her husband’s side during the press conference in which he made the comments, is accused of serious corruption but Mr Mugabe demanded critics “leave her alone”.

Several civil society groups are collating results from 10,985 polling posts in parallel with the ZEC, but are not allowed to release results before the commission. A source at one group said it was too early to call a winner but it was looking "very close".

Mr Mnangagwa's spokesperson, George Charamba, and Zanu-PF spokesperson Simon Khaya-Moyo did not respond to calls for comment.

In the capital Harare, an MDC stronghold, results posted outside some polling stations seen by Reuters showed Mr Chamisa winning by wide margins but Mr Mnangagwa was expected to claw back ground in the ruling Zanu-PF rural heartland.

Urban results tend to emerge quicker than those from rural outposts, where communication is poor.

In some rural constituencies in the east and south of the country, counting of votes was still ongoing but was expected to end early on Tuesday, some parliamentary candidates said.

Some 5.5 million people were registered to vote and dozens of people waited in line on Monday to vote outside many polling stations in Harare, the capital.

“I want to do this and get on with my business. I am not leaving anything to chance. This is my future,” said Emerina Akenda, a first-time voter.

In pictures: Zimbabwe votes in historic election Show all 55 1 /55 In pictures: Zimbabwe votes in historic election In pictures: Zimbabwe votes in historic election People in Mbare celebrate after officials announced the re-election of President Emmerson Mnangagwa Getty Images In pictures: Zimbabwe votes in historic election Supporters celebrate at the Harare International Convention Centre in the early hours Getty Images In pictures: Zimbabwe votes in historic election People in Mbare celebrate Getty Images In pictures: Zimbabwe votes in historic election People in Mbare celebrate Getty In pictures: Zimbabwe votes in historic election Supporters sing and dance soon after the announcement EPA In pictures: Zimbabwe votes in historic election Supporters sing and dance soon after the results of the Presidential elections were announced at the Harare International Convention Centre EPA In pictures: Zimbabwe votes in historic election A vendor sells sponges near newspaper headlines on the streets of Harare AP In pictures: Zimbabwe votes in historic election Supporters of the newly reelected Zimbabwe President Emmerson Mnangagwa celebrate in Mbare AFP/Getty Images In pictures: Zimbabwe votes in historic election People in Mbare celebrate Getty Images In pictures: Zimbabwe votes in historic election A man reads a Zimbabwean newspaper the day after the violence on the streets of Harare EPA In pictures: Zimbabwe votes in historic election A soldier fires shots towards demonstrators AFP/Getty Images In pictures: Zimbabwe votes in historic election A wounded man reacts at the sight of soldiers while taking shelter in a stall at a market in Harare AFP/Getty Images In pictures: Zimbabwe votes in historic election Supporters of the opposition Movement for Democratic Change party (MDC) light a fire in the streets of Harare. Reuters In pictures: Zimbabwe votes in historic election Soldiers open fire to disperse crowds of the opposition Movement for Democratic Change supporters outside the party's headquarters in Harare REUTERS In pictures: Zimbabwe votes in historic election Opposition MDC party supporters hold a voting placard up high on a street in Harare AP In pictures: Zimbabwe votes in historic election A supporter of Zimbabwean opposition MDC Alliance allegedly beaten up by soldiers sits with blood on his face as other protesters assist him AFP/Getty Images In pictures: Zimbabwe votes in historic election A vendor scurries for cover with her wares as soldiers disperse demonstrators AFP/Getty Images In pictures: Zimbabwe votes in historic election A wounded man hides under a tarpauling while the Zimbabwean Army patrols the streets of Harare AFP/Getty Images In pictures: Zimbabwe votes in historic election Anti-riot police officers close the entrance to the Rainbow Towers, where the election results were announced in Harare Getty In pictures: Zimbabwe votes in historic election Soldiers open fire on citizens REUTERS In pictures: Zimbabwe votes in historic election Citizens run the Zimbabwean army during protests in Harare Reuters In pictures: Zimbabwe votes in historic election Supporters of the opposition Movement for Democratic Change party (MDC) protest the result of the election, which they allege to be fraudulent Getty In pictures: Zimbabwe votes in historic election Supporters of the opposition protest in the streets of Harare as police fire tear gas AP In pictures: Zimbabwe votes in historic election A soldier assaults a man on the streets of Harare during protests AP In pictures: Zimbabwe votes in historic election Zimbabwean anti riot police officers sit in a truck amidst protesters on the streets of Harare Getty In pictures: Zimbabwe votes in historic election Citizens run from riot police amidst protests in Harare AP In pictures: Zimbabwe votes in historic election Soldiers patrol the streets during protests in Harare Reuters In pictures: Zimbabwe votes in historic election A Zimbabwean soldier beats a man on the streets of Harare Getty In pictures: Zimbabwe votes in historic election A man lies dead after the Zimbabwean army shot at protesters in Harare Getty In pictures: Zimbabwe votes in historic election A supporter of the opposition party protests in the streets of Harare Reuters In pictures: Zimbabwe votes in historic election Zimbabwean soldiers beats an opposition supporter on the streets of Harare Reuters In pictures: Zimbabwe votes in historic election Supporters of the opposition protest in the streets of Harare AP In pictures: Zimbabwe votes in historic election Supporters of the opposition protest in the streets of Harare as police fire tear gas Getty In pictures: Zimbabwe votes in historic election Supporters of the opposition burn a campaign banner of the ruling Zanu-PF party Reuters In pictures: Zimbabwe votes in historic election A man casts his vote in Mbare, a suburb of Zimbabwean capital Harare Getty In pictures: Zimbabwe votes in historic election A police officer keeps watch of queuing voters in Mbare Getty In pictures: Zimbabwe votes in historic election Voters stand in line outside a polling station in Mbare Getty In pictures: Zimbabwe votes in historic election Election officials remove political posters near a polling station on voting day Getty In pictures: Zimbabwe votes in historic election Oppostion leader Nelson Chamisa arrives at a polling station in Harare Getty In pictures: Zimbabwe votes in historic election Oppostion leader Nelson Chamisa casts his vote AP In pictures: Zimbabwe votes in historic election Zimbabwean president Emmerson Mnangagwa arrives to cast his vote in Kwekwe Reuters In pictures: Zimbabwe votes in historic election Zimbabwean president Emmerson Mnangagwa casts his vote AP In pictures: Zimbabwe votes in historic election Voters stand in line outside a polling station in Kwekwe AP In pictures: Zimbabwe votes in historic election A woman prays outside a polling station in Mbare Getty In pictures: Zimbabwe votes in historic election Voters arrive at a polling station in Harare EPA In pictures: Zimbabwe votes in historic election Former president Robert Mugabe prepares to cast his vote in Harare AP In pictures: Zimbabwe votes in historic election A woman casts her vote in Mbare, a suburb of capital Harare Getty In pictures: Zimbabwe votes in historic election A woman casts her vote in the township of Makokoba Getty In pictures: Zimbabwe votes in historic election A police officer casts his vote in Mbare Getty In pictures: Zimbabwe votes in historic election A woman prays near a polling station in Mbare EPA In pictures: Zimbabwe votes in historic election A man casts his vote in Harare AP In pictures: Zimbabwe votes in historic election Former president Robert Mugabe speaks at a press conference the day before the vote where he stated that he would not be voting for his former Zanu-PF party Reuters In pictures: Zimbabwe votes in historic election A supporter of the opposition MDC party attends the final campaign rally in Harare on July 28 Reuters In pictures: Zimbabwe votes in historic election Supporters of the opposition MDC (Movement for Democratic Change) party attends the final campaign rally in Harare on July 28 EPA In pictures: Zimbabwe votes in historic election Supporters of the opposition MDC (Movement for Democratic Change) party attends the final campaign rally in Harare on July 28 EPA

“We need change because we have suffered a lot here,” said 65-year-old Mable Mafaro while voting in Harare. “We have suffered a lot. That’s all.”

The election winner faces the task of putting Zimbabwe back on track after 37 years of rule by Mr Mugabe, a period tainted by corruption, mismanagement and diplomatic isolation that caused crises in a country that once had one of Africa's most promising economies.

A credible vote is essential if Zimbabwe is to exit painful sanctions and secure the donor funding and investment needed to stem chronic cash shortages.

The run-up to Monday's vote was largely peaceful compared to past elections under Mr Mugabe, where the ruling party and war veterans were accused of violence against opponents.

Dozens of people were killed ahead of a runoff in 2008 between Mr Mugabe and MDC-founder Morgan Tsvangirai, who died of cancer in February.

Mr Mugabe emerged on the eve of the election to announce he would vote for the opposition, surprising Mr Mnangagwa who accused him of striking a deal with Mr Chamisa.