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.
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 Organization 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 3,000 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 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.