Also absent from the UN health body's conference was China, where the virus surfaced.
French President Emmanuel Macron, German Chancellor Angela Merkel and South African President Cyril Ramaphosa were among those who joined in the launch of what the WHO has billed as a "landmark collaboration" to fight the pandemic and ensure equal access to treatments for rich and poor.
"We are facing a common threat which we can only defeat with a common approach," WHO Director General Tedros Adhanom Ghebreyesus said as he opened the virtual meeting.
"Experience has told us that even when tools are available they have not been equally available to all. We cannot allow that to happen."
UN Secretary General Antonio Guterres said: "A world free of Covid-19 requires the most massive public health effort in history."
The appeal came a day after the US president prompted an outcry with his suggestion that heat or sunlight could weaken Covid-19, or that industrial cleansers could be used to treat patients.
"Is there a way we can do something like that, by injection inside or almost a cleaning?" Trump mused during a televised briefing. "It sounds interesting to me."
s experts -- and disinfectant manufacturers -- rushed to caution against any such dangerous experiment, the president tried to walk back his comments claiming he had been speaking "sarcastically."
The hardest-hit country by far in the global pandemic, the US had on Saturday recorded 52,217 deaths and more than 925,758 infections.
Confirmed coronavirus were at 2.8 million worldwide, with deaths at 197,368.
New reported cases seem to have levelled off at about 80,000 a day, as distancing measures have taken root and the daily death toll in Western countries appeared to be falling -- a sign hopeful epidemiologists have been looking for.
Yet other nations are still in the early stages of the fight and the World Health Organization has warned strict measures should remain in place.
New cases were accelerating in countries with low testing or late and limited mitigation like Russia, Sudan, Somalia, Nigeria, the Maldives and Guatemala.
In Argentina, with some 3,400 cases and 167 deaths so far, prisoners rioted and demanded their release from a Buenos Aires jail on Friday after confirmation of a coronavirus case inside the facility.