Tourist arrivals fell by 22 percent in the first three months of the year, and by 57 percent in March alone, with Asia and Europe suffering the biggest declines, according to the Madrid-based organisation.
"The world is facing an unprecedented health and economic crisis. Tourism has been hit hard, with millions of jobs at risk in one of the most labour-intensive sectors of the economy," the body's secretary general, Zurab Pololikashvili, said.
Airlines have suffered the most since the outbreak began in China in late 2019 with most flights grounded, but hotel groups, cruise operators and tour operators are also reeling.
The UN body had forecast at the beginning of the year that international tourism would grow by 3.0-4.0 percent in 2020 but then revised its forecast at the end of March, predicting a 20-30 percent decline.