A major incident has been declared in London and more than 10,000 new Omicron cases have been confirmed in the UK, as the variant surges across the country.
A further 90,418 daily Covid cases have been reported across the UK on Saturday, after days of record highs.
Cabinet ministers have been briefed on the latest Covid data.
Sadiq Khan said Friday’s 26,000 new cases in London were having an impact on staff absences for the capital’s emergency services.
“I’ve been meeting over the last few days, on a daily basis, colleagues across the city from the NHS to councils, from the fire service to the police – we’re incredibly concerned by the huge surge in the Omicron variant,” he said.
“The big issue we have is the number of Londoners who have this virus and this is leading to big issues in relation to staff absences and the ability of our public services to run at the optimal levels.
“I’ve taken the decision, in consultation with our partners, to declare a major incident. It’s a statement of how serious things are.”
Meanwhile, police officers suffered minor injuries during “scuffles” at a protest against coronavirus restrictions at Westminster.
Latest government data has shown there are 1,534 Covid patients in London hospitals – up 28.6% on last week – with about 200 new admissions per day.
Saturday’s data – which included the second-highest number of cases since mass testing began last year – also saw another 125 deaths recorded within 28 days of a positive test, down slightly on a week ago.
Omicron is now thought to now be the dominant variant in England and Scotland, replacing Delta.
Over 800,000 booster doses were reported for the second consecutive day on Saturday, with 817,625 third jabs.
Meanwhile, epidemiologist Prof Neil Ferguson, who is a government adviser but was not involved in Thursday’s Sage meeting, said the true number of infections was likely to be much higher than those reported – and might be 300,000 a day.
On hospital admissions, he said there had been a significant surge in the London region, which was ahead of the rest of the country in terms of the spread of Omicron.
Speaking to the BBC Radio 4’s Today programme in a personal capacity, he also said there were concerns “we’ll be heading into something which has the risk of overwhelming the public health service”.
The Liberal Democrats said the government needed to “come clean with the public about what Omicron means for Christmas”.
The party’s health spokesperson Daisy Cooper said: “Ministers must act now to protect NHS staff and ensure that urgent NHS services are available to everyone over the Christmas period.
“The government should explain what additional public health protections are needed to bring the NHS back from the brink of collapse, and urgently get support to struggling businesses.”
Hannah Essex, co-exec director at the British Chamber of Commerce, told BBC Breakfast further restrictions would need to come with a package of support for businesses.
She said the past 20 months had been “absolutely brutal” for businesses and doing nothing was not an option.
“They were just starting to see things pick up and this is the point at which we could see them fall over the edge. And that would be such a terrible shame and will inevitably lead to job losses as well,” she added.