"The impacts of climate change are a clear and present danger, and its most immediate and lethal manifestation is extreme heat," said Hans Henri Kluge, the WHO's Europe director.
"Heat is a silent killer, but it is not an inevitable one," Kluge said at an event in Berlin to launch new guidelines on protecting lives from extreme heat.
Extreme heat affects especially the very old and the very young and people with heart, kidney and other diseases by causing dehydration, heatstroke and exacerbating existing conditions.
Most of the 200,000 deaths were "entirely preventable", Kluge said, and the number was "the tip of the iceberg, with millions more people being affected physically and mentally".
Scientists say human-driven climate change is amplifying extremes, with weather events like heatwaves, droughts and floods becoming more intense and frequent.
Kluge said that Europe was "warming faster than any other continent".
The WHO's guidance urges authorities to put in place effective heat-warning systems and communication with vulnerable groups.
It calls for more action to reduce exposure to extreme heat through urban planning measures such as creating and maintaining more green spaces.
Other advice includes social services checking that older people stay hydrated, and changing the shifts of workers so they can avoid the midday sun.
While individual action such as staying out of the heat was important, "it is not enough to fight a systemic crisis" Kluge said, calling for a "coordinated, powerful and institutional response".
In late May a swathe of western Europe saw a record-breaking early summer heatwave in what the UN's climate chief Simon Stiell called "a brutal reminder of the spiralling impacts of the climate crisis".
Authorities in Spain said last week that this year they had recorded the highest number of heat-related deaths for the month of May since 2015.
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