The home secretary has urged members of the public to call the police on their neighbours if they see breaches of coronavirus restrictions.
The new rules have been criticised for not being clear, for unusual clauses - such as a ban on "mingling" - and for the long list of exemptions.
The "rule of six", which came into force on Monday, restricts people to gatherings of six people or less and police have powers to break up larger groups.
Asked whether people should report their neighbours over breaches, Priti Patel said: "Where people are being non-compliant, of course, they will absolutely do what they consider to be the right thing and look for enforcement.
"Illegal gatherings should have enforcement undertaken," she added, "which is why police have the powers to issue fixed penalty fines.
"This is not new, police have been enforcing through coronavirus regulations, for this very purpose to stop the spread of coronavirus."
People face fines of £100, doubling to a maximum of £3,200 for repeat offences, for breaching the law, which bans social gatherings of more than six people both indoors and outdoors.
Outdoor “sports gatherings” are exempt from the restrictions, meaning grouse shooting and hunting with guns in England in groups up to 30 can continue.
Rules allow for more than six people in total in indoor settings "operated by a business, a charitable, benevolent or philanthropic institution or a public body".
They can also gather at outdoor events organised by "a business, a charitable, benevolent or philanthropic institution, a public body, or a political body".
But those attending must be part of a "qualifying group" of up to six people, a single household or linked households.
And no person is allowed to become a member of another group or "otherwise mingle" with anyone outside their own group, according to the legislation.
Human rights barrister Adam Wagner tweeted: "It's illegal to mingle! What does mingle mean?
"Is saying hello to someone at a gathering 'mingling'? What about holding the door open for them?"
Other exceptions listed in the legislation include attending a "support group", "significant event gathering", Covid-secure weddings or funerals.
Protests are also allowed as long as a risk assessment is carried out and guidance followed.
The Health Protection (Coronavirus, Restrictions) (No. 2) (England) (Amendment) (No. 4) Regulations 2020 also states more than six people can gather from the same household or two "linked households" - made up of a household of one adult and any number of children and a second with no limit on the numbers of adults or children.
Guidance for police was only published by the government on Sunday, hours before the new rules came intro force.
Policing minister Kit Malthouse said on Monday anyone who was concerned about too many people gathering in one place, should consider reporting it.
"If they think there is a gathering of concern, they should call the non-emergency number and let police know," he said.
He added that 'Covid marshals' would be explaining and advising potential rule-breakers about the new guidelines.
Mr Malthouse said the marshals had been working in Leeds and Cornwall "very well" but that if necessary, police would be called in to deal with potential illegal gatherings.