People in Leeds have been warned not to attend any large gatherings after six events were cancelled due to the pandemic including Leeds Festival.
Police said it was concerned raves and other gatherings were being organised instead for the bank holiday weekend.
Officers said a number of unlicensed events were held during lockdown, which were banned under current restrictions.
Leeds City Council recorded a total of 4,068 coronavirus cases with a rate of 512.9 per 100,000 population.
But it logged 114 positive tests in the week to 10 August compared with 132 the previous week.
Major events including Leeds West Indian Carnival, Leeds Pride and the Black Music Festival are usually planned for the August Bank Holiday but this year had to be postponed.
Assistant Chief Constable Tim Kingsman, of West Yorkshire Police, urged people to act responsibly and said attending unlicensed events and large gatherings presented "a real risk to everyone's health".
"We are working hard alongside our partner agencies to keep the public safe from this significant and deadly threat to public health and we will continue those efforts throughout the bank holiday weekend and beyond.
"We will be maintaining our approach of engaging, explaining and encouraging compliance with the restrictions in the first instance but will look to enforcement measures where necessary."
Mr Kingsman said he hoped people would "take this message on board and do the right thing for themselves and their communities".
Councillor Debra Coupar said: "We are not trying to spoil anybody's fun, but we are asking that people continue to be sensible and follow the government guidance regarding coronavirus to help stop the spread.
"This means not attending any large gatherings, unlicensed events or activities where social distancing cannot be maintained to ensure that everybody's health and wellbeing is being protected and not put at risk."