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.
London's mayor said he was "incredibly concerned" by the city's infection levels and the major incident was "a statement of how serious things are".
Sadiq Khan said Friday's 26,000 new cases in London were having an impact on staff absences for the capital's emergency services.
He said that NHS trusts, councils, the fire service, police and City Hall were "incredibly concerned by the huge surge in the Omicron variant".
In this context, the major incident status would allow organisations to work more closely together, he added.
Meanwhile, police officers suffered minor injuries during "scuffles" at a protest against coronavirus restrictions at Westminster.
Latest government data has shown there are 1534 Covid patients in London hospitals - up 28.6 percent 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.
The World Health Organisation has said the Omicron variant has been identified in at least 89 countries - and is spreading significantly faster than the Delta strain.
It is spreading rapidly in countries with high levels of population immunity, it added.
In England, modelling indicates hospital admissions could peak at at least 3000 a day without intervention measures beyond the Plan B rules currently in place, advisers from the Scientific Advisory Group for Emergencies (Sage) said in the leaked minutes of a meeting held on Thursday.
The current Plan B rules for England include Covid-19 passes for certain events, face masks in more places and people being urged to work from home if they can.
The other nations of the UK have already brought in similar rules - and Scotland has gone further by asking people to limit social contact to three households at a time in the run-up to Christmas. Wales has also ordered nightclubs to close from 27 December.
Omicron is now thought to 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.