The main impact of the move to "high" is that people cannot meet other households socially indoors in any setting, for example at home or in a restaurant. Travel should be reduced where possible, Health Secretary Matt Hancock said.
Anger, though, is rising over the economic, social and health costs of the biggest curtailment of freedoms since World War Two. One former government adviser warned some people would have trouble clothing their children soon.
People working in the hospitality sector are especially concerned about the effect the harsher restrictions will have on their livelihoods.
London is not the only major UK city to have been placed on 'high' alert. The city of York, in Northern England, will also be placed under the same restrictions.
Prior to the restrictions coming in, Londoners still took the opportunity to take advantage of time together in social settings.