The mayor hoped that the declaration would help to fast-track help for the National Health Service (NHS), which he described as "on the cusp of being overwhelmed" and running out of beds.
Mayor of London Sadiq Khan speaks during a vigil at Guildhall Yard in London on Monday to remember the London Bridge stabbing victims and honor members of the emergency services and bystanders who fought the attacker. (AP)
"If we do not take immediate action now, our NHS could be overwhelmed and more people will die," Khan said in a statement. "Londoners continue to make huge sacrifices and I am today imploring them to please stay at home unless it is absolutely necessary for you to leave."
"Stay at home to protect yourself, your family, friends and other Londoners and to protect our NHS."
The declaration does not provide the mayor or authorities with additional powers, but it provides for easier coordination and ability to demand more help from Westminster, according to the Guardian.
More than anything, the move is seen as a symbolic one to make clear to London residents just how severe the situation in their city is.
Khan told BBC reporter Karl Mercer that the number of Londoners with COVID-19 is as high as 1 in 20 in some parts of London, while the Office for National Statistics lists the estimate at 1 in 30.
Khan also claimed that there are more than 7,000 people in hospital with COVID-19, which is a 35% increase over the previous peak.