'MOST SIGNIFICANT VARIANT'
Several other countries including India, Japan, Israel, Turkey, Switzerland and the United Arab Emirates also toughened travel curbs.
In Geneva the WHO - whose experts on Friday discussed the risks that the variant, called B11529, presents - had earlier warned against travel curbs for now.
"It's really important that there are no knee-jerk responses here," said the WHO's emergencies director Mike Ryan, praising South Africa's public health institutions for picking up the new variant of the coronavirus that causes COVID-19.
The variant has a spike protein that is dramatically different to the one in the original coronavirus that vaccines are based on, the UK Health Security Agency said, raising fears about how current vaccines will fare.
"As scientists have described, (this is) the most significant variant they've encountered to date," British Transport Secretary Grant Shapps told Sky News.
On Wall Street the Dow Jones Industrial Average was down 2.5 per cent, tracking its worst day since late October 2020, and European stocks had their worst day in 17 months as financial markets digested the news.
Cruise operators Carnival Corp, Royal Caribbean Cruises and Norwegian Cruise Line plunged more than 9 per cent each, while shares in United Airlines, Delta Air Lines and American Airlines slumped almost 10 per cent.
TOO LATE FOR TRAVEL CURBS?
The coronavirus has swept the world in the two years since it was first identified in central China, infecting almost 260 million people and killing 5.4 million.
One epidemiologist in Hong Kong said it may be too late to tighten travel curbs against the latest variant.
"Most likely this virus is already in other places. And so if we shut the door now, it's going to be probably too late," said Ben Cowling of the University of Hong Kong.
Belgium identified Europe's first case, adding to those in Botswana, Israel and Hong Kong.
Brazilian health regulator Anvisa recommended that travel be restricted from some African countries, but President Jair Bolsonaro appeared to dismiss such measures.
Bolsonaro has been widely criticised by public health experts for his management of the pandemic, railing against lockdowns and choosing not to get vaccinated. Brazil has the world's second-highest death toll from the virus, behind only the United States.
Discovery of the new variant comes as Europe and the United States enter winter, with more people gathering indoors in the run-up to Christmas, providing a breeding ground for infection.
Friday also marked the start of the holiday shopping period in the United States, when retailers offer discounts. This year, US shoppers found stores less crowded than in years past.
Realtor Kelsey Hupp, 36, was at the Macy's department store in downtown Chicago on Black Friday.
"Chicago is pretty safe and masked and vaccinated. I got my booster so I'm not too concerned about it," she said.