Its Director-General, Dr Tedros Ghebreyesus, gave the advice during a news briefing from Geneva on Monday.
Ghebreyesus said he had observed the eagerness of countries to “get their economies going again”, eight months into the coronavirus pandemic.
According to him, if countries are serious about opening, they must prioritise suppression of transmission and safety of lives.
He warned that opening up without efficient and effective prevention measures in place was “a recipe for disaster”.
Ghebreyesus said this might seem an impossible balance, but it could be done if countries were in control of transmission.
“The more control they have, the more they can open. The reality is that coronavirus spreads easily.
“It can be fatal for people of all ages and most people remain susceptible.
“To control transmission, it is essential to prevent events that lead to outbreaks,” he said.
The virus spreads efficiently among clusters of people, the WHO boss stated, adding that “explosive outbreaks” are linked to gatherings at sports stadiums, nightclubs and places of worship.
While noting that there were ways to hold gatherings safely, Ghebreyesus said decisions about how and when must be made with a risk-based approach, tailored to local conditions.
He said countries experiencing significant community transmission might need to postpone such events.
Those seeing sporadic cases or small clusters, on the other hand, can find creative ways to hold events while minimising risk.
He called for emphasis on reduction of deaths by protecting the elderly, people with underlying conditions and essential workers.
“Countries that do this well may be able to cope with low levels of transmission as they open.
“Individuals must play their part by staying at least one metre away from others, cleaning their hands regularly, practicing respiratory etiquette by wearing a mask and avoiding close-contact settings.
“For governments, widespread stay-at-home orders can be avoided if they take temporary, geographically targeted interventions.
“It is important to find, isolate, test and care for COVID-19 cases – and both trace and quarantine contacts,” he said.