The impact of the rapid spread of the variant was already felt across the city with staff absences in front-line services, a statement from the mayor's office said.
Khan said the surge was “hugely concerning.” Urging citizens to get vaccinated, he added that the omicron “variant has quickly become dominant with cases increasing rapidly and the number of patients in our hospitals with Covid-19 on the rise again.”
The declaration was made after 65,525 new Covid-19 cases were confirmed in London over the past seven days, the statement said. In the last week, the number of coronavirus patients in London hospitals has gone up 29 percent, it added.
A major incident means coordination arrangements between key public services will be stepped up, and the city will be able to seek further support from the government to address the pressures caused by the virus's spread, the statement said.
The announcement will help authorities reduce service disruption and allow more time to administer booster vaccines, as experts learn more about the the severity of the variant and the impact it will have on the National Health Service, it added.
The news comes as the United Kingdom reported more than 93,000 cases on Friday alone, breaking the record for a third day in a row.
New regulations intended to help slow the spread of omicron, including masks in public places and the use of Covid-19 passes for some venues, passed in Parliament earlier this week.
As of Thursday, the omicron variant had been detected in 89 countries, according to the World Health Organization, which said it was spreading significantly faster than the delta variant in countries with documented community transmission, with a doubling time between 1 1/2 and three days.
The severity of the omicron variant is still unclear, but hospitalizations in the U.K. and South Africa continue to rise, and given rapidly increasing case counts, it is possible many health care systems may become quickly overwhelmed, the WHO said Saturday.
Meanwhile, in the United States, some schools were closing their doors once again, moving back to online learning as they braced for the rapid rise in omicron cases amid delta's continued onslaught. At the same time, people are clamoring to get their hands on coronavirus tests as Americans prepare to travel for holiday gatherings.
This week, the number of coronavirus deaths in the U.S. passed 800,000.