The ‘full rule of six’ restrictions which ban large social gatherings have been outlined just minutes before they went into force.
The new coronavirus law came into force in England at midnight.
People who flout the new restrictions face fines of up to £3,200 if they fail to abide.
The rule applies to both indoor and large social outdoor settings and follows a rapid increase in the number of daily positive cases.
Regulations enabling the enforcement of the rule were published late on Sunday night, just minutes before they came into effect.
More than 3,000 Covid-19 cases were recorded in the UK for the third day in a row on Sunday – the first time since May that cases were above 3,000 on three consecutive days.
The new law comes amid concerns about an increase in cases in care homes and growing criticism of the NHS Test and Trace system.
Aside from limited exemptions including work and education, police will be able to disperse gatherings of more than six people and issue fines ranging from £100 to £3,200.
Home Secretary Priti Patel said the recent rise in cases “makes it clear that more needs to be done to stop the spread of this disease”.
The rule applies across England and replaces the existing ban on participating in gatherings of more than 30 and the current guidance on allowing two households to meet indoors.
Chair of the National Police Chiefs’ Council Martin Hewitt has urged the public to “stick to the limits”.
He said: “Preventing the spread of coronavirus is a shared effort and police are playing our part alongside Government, businesses, hospitality owners, local authorities and others.
“Officers are in their communities following our approach to engage, explain and encourage people to follow the rules. We will issue fines when people refuse to comply.
“The demands on the police service are now at similar levels to before the pandemic, which makes it crucially important that we all take personal responsibility, stick to the limits and prevent the spread of this deadly virus.”
Meanwhile, a survey suggests a second peak is the number one concern among medics who want to avoid a return to the “horror and tragedy” of the pandemic’s early days.
The British Medical Association (BMA) poll found that 86% of more than 8,000 doctors and medical students in England said that a second peak was likely or very likely in the next six months.
Dr Chaand Nagpaul, BMA council chairman, said: “With daily cases still alarmingly high, and winter just around the corner, we are at a critical crossroads in the fight against this deadly virus.
“All efforts must be made to avoid a repeat of the horror and tragedy we all experienced earlier this year.”
Officials will be hoping the warm and sunny weather forecast for Monday does not encourage people to gather in groups in outdoor spaces.
Temperatures as high as 29C (84F) are expected in some parts, according to the Met Office.
The Government said that as of 9am on Sunday, there had been a further 3,330 lab-confirmed cases of coronavirus in the UK.
Tough new Covid-19 lockdown measures were announced for parts of the UK on Friday as cases continued to rise and as the R number – the reproduction number of coronavirus transmission – climbed above one.
According to Government advisers, the last time R was above one was in early March.
How are the new rules being applied in each nation?
The new rules are being applied slightly differently in each devolved administration.
From Monday, gatherings of more than six people are illegal.
The rules apply across England to all ages and in any setting either indoors and outdoors, at home or a pub.
A single household or support bubble that is larger than six will still be able to gather.
Covid-secure venues like places of worship, gyms, restaurants and hospitality settings can still hold more than six in total.
Education and work settings are not affected by the new rules.
Weddings and funerals can still go ahead with a limit of 30 people if conducted in a Covid-secure way.
People in Wales are only able to meet in groups of six or under indoors and must all belong to the same extended household group.
Up to four households can join together to form an extended household.
But, unlike in England, children under 12 are exempt and will not count towards that total.
Also unlike in England, people can still meet up in groups of up to 30 outdoors, as long as social distancing is maintained.
The changes do not apply in Caerphilly county borough due to its rise in Covid-19 cases.
A maximum of six people from two households are allowed to meet together in Scotland.
Just like in England, the new limit applies when people meet in restaurants, pubs and beer gardens, as well as in homes.
However, children under the age of 12, who are part of the two households meeting do not count towards the limit of six people, First Minister Nicola Sturgeon has said.
There are “some limited exceptions”, covering organised sports and places of worship.
Up to 20 people can attend weddings, civil partnerships and funerals, as well as receptions and wakes, which is more stringent than both England and Wales.
Northern Ireland has not announced any changes to how many people can gather. However, localised coronavirus restrictions were introduced in Belfast and Ballymena.
People from two or more households in these areas are not able to meet in private settings.
There are a number of limited exceptions, including childcare provision and households that have formed a social bubble with another.
No more than six people, from no more than two households, are allowed to meet in private gardens.
In Northern Ireland, the number of people who can gather indoors in a private home was already reduced from 10 people from four households to six people from two households last month due to a rise in Covid-19 cases.
Up to 15 people can meet outdoors.