In a statement, Mr Khan said: “The situation in London is now critical with the spread of the virus out of control.
“Our heroic doctors, nurses and NHS staff are doing an amazing job, but with cases rising so rapidly, our hospitals are at risk of being overwhelmed.
“The stark reality is that we will run out of beds for patients in the next couple of weeks unless the spread of the virus slows down drastically.
“We are declaring a major incident because the threat this virus poses to our city is at crisis point.
“If we do not take immediate action now, our NHS could be overwhelmed and more people will die.
“Londoners continue to make huge sacrifices and I am today imploring them to please stay at home, unless it is absolutely necessary.”