He sounded the alarm about the "inaction" of countries in response to the disease and urged a better reaction to "the alarming levels of spread and severity" of the outbreak.
"Pandemic is not a word to use lightly or carelessly. It is a word that, if misused, can cause unreasonable fear, or unjustified acceptance that the fight is over, leading to unnecessary suffering and death," he said.
"Describing the situation as a pandemic does not change WHO's assessment of the threat posed by this coronavirus. It doesn't change what WHO is doing, and it doesn't change what countries should do.
He added: "We have never before seen a pandemic sparked by a coronavirus. And we have never before seen a pandemic that can be controlled at the same time. WHO has been in full response mode since we were notified of the first cases.
"We have called every day for countries to take urgent and aggressive action. We have rung the alarm bell loud and clear."
The WHO had already declared the outbreak a global health emergency, in January, as cases grew in China. In recent weeks, speculation has turned to when the organisation would declare it a pandemic, amid rapidly growing deaths and infections.
The disease has killed more than 4,000 people around the world. It has infected more than 120,000 people worldwide and reached at least 114 countries.
The declaration of a pandemic is an official recognition that the disease has spread around the world beyond expectations. It suggests the disease is likely to spread around communities and that governments should act to prepare.
It is used in contrast with an epidemic, which is used when a disease is largely limited to one country or community. While many had expected the WHO to declare a pandemic, it marks a recognition that the disease is spreading quickly and widely.
Large outbreaks have not been declared pandemics in the past. The Sars coronavirus that struck in 2003, for instance, was never declared a pandemic because its spread was largely limited to a smaller number of countries.
The official use of the word does not necessarily have strict criteria, and so relies on the discretion of senior officials at the WHO. As well as an official recognition of the spread of the disease, it can be used by the organisation to call on governments and officials around the world to begin a more co-ordinated and concentrated response.
The World Health Organization's use of the word pandemic to describe the outbreak of the new coronavirus does not change its response, the head of the WHO's emergencies programme said.
Dr Mike Ryan also told a news conference the situation in Iran was "very serious" and the agency would like to see more surveillance and more care for the sick.
"This decision will likely have been made on the basis of the majority of the world’s continents now seeing significant and ongoing person to person spread of SARS-COV2," said Nathalie MacDermott from King’s College London. "The change of term does not alter anything practically as the world has been advised for the last few weeks to prepare for a potential pandemic, which has hopefully been taken seriously by all countries.
"The use of this term however highlights the importance of countries throughout the world working cooperatively and openly with one another and coming together as a united front in our efforts to bring this situation under control.”
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies