Dr Tedros Adhanom Ghebreyesus urged countries to use aggressive measures, community mobilisation and social distancing to save lives.
“Do not just let this fire burn,” he said.
His comments came as several European countries reported steep rises in infections and deaths. Italy has recorded its highest daily toll yet.
There were 250 deaths recorded over the past 24 hours, taking the total to 1,266, with 17,660 infections overall.
Spain, the worst-affected European country after Italy, reported a 50% jump in fatalities to 120 on Friday. Infections increased to 4,231.
Prime Minister Pedro Sanchez says a state of alert will come into effect there on Saturday for two weeks.
Why is Europe the ‘epicentre’?
More than 132,500 people have been diagnosed with Covid-19 in 123 countries around the world, according to the WHO.
The total number of deaths has reached about 5,000 – a figure Dr Tedros described as “a tragic milestone”.
“Europe has now become the epicentre of the pandemic, with more reported cases and deaths than the rest of the world combined, apart from China,” he said.
“More cases are now being reported every day than were reported in China at the height of its epidemic.”