The spread of Covid in London is "out of control" according to Sadiq Khan, who has declared a "major incident".
The coronavirus infection rate in London has exceeded 1,000 per 100,000 people, based on the latest figures from Public Health England.
However, the Office for National Statistics recently estimated as many as 1 in 30 Londoners has coronavirus.
Mr Khan told BBC political reporter Karl Mercer that the figure is as high as 1 in 20 in some parts of London.
Major incidents have previously been called for the Grenfell Tower fire in June 2017 and the terror attacks at Westminster Bridge and London Bridge.
Such an incident is defined as an event or situation with a range of serious consequences which requires special arrangements to be implemented by one or more emergency responder agency.
Currently, there are more than 7,000 people in hospital with Covid-19, the mayor said.
This is a 35% increase compared to the previous peak of the pandemic, Mr Khan added.
The mayor of London's announcement comes after the counties of Sussex and Surrey declared similar major incidents on Thursday.
He said the London Ambulance Service was currently taking up to 8,000 emergency calls a day, compared to 5,500 on a typical busy day.
It comes after the London Fire Brigade said more than 100 firefighters had been drafted in to drive ambulances to help cope with the demand.
'Cut contacts'
London's regional director for Public Health England Kevin Fenton said the current wave of coronavirus was "the biggest threat" the capital has faced in this pandemic to date.
Prof Fenton urged Londoners to "cut contacts" and "reduce movements".
He added: "The emergence of the new variant means we are setting record case rates at almost double the national average, with at least one in 30 people now thought to be carrying the virus.
"We know this will sadly lead to large numbers of deaths, so strong and immediate action is needed."
Mr Khan is warning that London was "at crisis point".
"If we do not take immediate action now, our NHS could be overwhelmed and more people will die," he said.
"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."
He said he had written to Prime Minister Boris Johnson asking for more financial support for Londoners who need to self-isolate and are unable to work, and for daily vaccination data.
Analysis
BBC London political reporter Karl Mercer
The mayor of London was in a sombre mood when I spoke to him earlier this afternoon. One in 20 Londoners in some areas now has Covid, and there is a real fear that hospitals will simply be overwhelmed in the next two weeks.
Declaring a major incident is a real indication of the levels of concern felt not just at City Hall but across London's emergency services and the NHS.
More Londoners are now in hospital with coronavirus than at the peak of the first wave last April - and those numbers are growing by more than 800 every day.
It's believed the last mayor to declare a London-wide major incident was Boris Johnson in response to the 2011 riots.
The man who replaced him at City Hall has now written to the prime minister asking him to enforce mask-wearing in all busy public spaces, and to ban public worship.
The coming days will be some of the most challenging in the city's recent history.
Georgia Gould, chair of London Councils, described London's rising coronavirus rate as "dangerous".
She added: "One in 30 Londoners now has Covid. This is why public services across London are urging all Londoners to please stay at home except for absolutely essential shopping and exercise.
"This is a dark and difficult time for our city but there is light at end of the tunnel with the vaccine rollout. We are asking Londoners to come together one last time to stop the spread - lives really do depend on it."
Earlier this week as the prime minister introduced an England-wide lockdown, the Met Police said officers were going to be "more inquisitive" towards Londoners seen outside.
The Met handed out 1,761 fines for breaches of coronavirus laws between 27 March and 20 December.
Downing Street has been approached for comment.
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https://news.google.com/__i/rss/rd/articles/CBMiNWh0dHBzOi8vd3d3LmJiYy5jby51ay9uZXdzL3VrLWVuZ2xhbmQtbG9uZG9uLTU1NTg4MTYz0gE5aHR0cHM6Ly93d3cuYmJjLmNvLnVrL25ld3MvYW1wL3VrLWVuZ2xhbmQtbG9uZG9uLTU1NTg4MTYz?oc=5
2021-01-08 14:05:00Z
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