The 65-year-old's symptoms came on suddenly while at a meeting of his main opposition Movement for Democratic Change (MDC) on Thursday evening in Kadoma, a city around 160 km (100 miles) southwest of the capital Harare.
Zimbabwe's main opposition leader and former prime minister, Morgan Tsvangirai, has been hospitalised in South Africa after his health deteriorated, a senior party official told AFP Saturday.
"He is in South Africa on account of a medical cause. He is being attended to," a senior official from Tsvangirai's Movement for Democratic Change (MDC) told AFP, speaking on condition of anonymity.
"We are monitoring the situation. He will be fine, it's only that he was overwhelmed with work and his health deteriorated."
The MDC spokesman could not be reached for comment.
Tsvangirai, 65, announced last year that he had been diagnosed with cancer of the colon and that he had begun chemotherapy.
The News Day, an independent daily, said Tsvangirai was airlifted to South Africa early Friday morning after he fell ill.
"He was on oxygen and drip and had been vomiting heavily," the paper said in its report.
Zimbabwean President Robert Mugabe, 93, Tsvangirai's rival for over a decade, regularly flies to Singapore, reportedly for medical reasons. Most details about his health have never been confirmed.
Tsvangirai's party has been riven by divisions since he struck a troubled four-year power-sharing deal with Mugabe after violent and disputed elections in 2008.
The former trade union leader has often accused Mugabe and his government of rigging polls.
In 2008 Tsvangirai won 47.9 percent of the vote to Mugabe's 43.2 percent, which led to a run-off.
But Tsvangirai pulled out before the final round of voting after a spate of violence against his supporters.