The Metropolitan Police has warned people not to gather in large groups over the Easter weekend after coronavirus restrictions were loosened.
Since the start of the week, the "rule of six" has returned in England.
And the capital is forecast to have a mild bank holiday break – fuelling concerns that people may gather in larger than half-dozen groups.
In northern parts of the country, the weather is expected to be chillier – with strong winds and snow predicted for some areas.
Under current COVID guidelines, groups of six or two households are allowed to meet outside.
But the Met says its officers will continue to shut down large gatherings like raves and house parties.
Deputy Assistant Commissioner Jane Connors said: "We cannot allow the selfish actions of a small minority of people to jeopardise the efforts of this city.
"We will continue to shut down house parties or dangerous raves quickly, taking enforcement action by handing out fines.
"We make no apology for our tough stance on shutting down those large gatherings which pose a serious risk to public health."
She added: "With nice weather on the way, we can expect to see parks and beauty spots busier than usual. Please stay safe and continue to follow the rules.
"Don’t crowd in open spaces and do follow the rule of six, or, only meet up with one other household."
Protests are also expected in the capital this week.
A statement from the force said: "While gathering for the purpose of a protest can be exempt, organisers must submit a risk assessment and take clear steps to make their gathering safe.
"Enforcement action will be taken, if needed, in the interests of public health."
On Saturday, temperatures in the South East and London are expected to be about 12C (54F).
Manchester and Leeds could see highs of 13C (55F) and 10C (50F) respectively.
By Monday, the capital may drop to 8C (46F), while Manchester could hit 7C (45F) and Leeds 5C (41F).