A new four-week lockdown has begun in England, with people told to stay at home and non-essential shops, pubs and gyms ordered to close.
The new rules ban households from mixing indoors or in private gardens, unless in a support bubble.
Police have warned that people who commit the most "egregious" breaches of the rules will face stiff fines.
The lockdown will "expire automatically" on 2 December, Prime Minister Boris Johnson has said.
On Wednesday, MPs backed the new lockdown which aims to combat a surge in coronavirus cases.
The lockdown, which came into effect at midnight, replaces the three tiers of regional restrictions that were previously in place across England. MPs are set to vote on the next steps needed to tackle the virus before the four weeks are up on 2 December.
Mr Johnson told MPs that a second lockdown was "not something any of us wanted to do" but insisted the restrictions represented "the best and safest path for our country".
On Wednesday, the UK recorded a further 492 coronavirus deaths - the highest daily figure since 19 May - and 25,177 confirmed cases.
Under the new restrictions, people should stay at home except for specific reasons including education and work (if it cannot be done from home).
All non-essential retailers, leisure and entertainment venues must shut, with pubs and restaurants told to close except for takeaways.
Martin Hewitt, chairman of the National Police Chiefs Council, warned that those responsible for the most "egregious" breaches of the rules would face stiff fines.
He said: "Every one of us has a responsibility to understand what the regulations are and abide by those regulations", adding that people who flouted them would be "breaking the law and endangering people's lives".
Chancellor Rishi Sunak is set to make a statement in the House of Commons later on Thursday outlining what economic support will be available to businesses and jobs during the lockdown.
Shoppers in England flocked to stores to make last-minute purchases on Wednesday, with some retailers extending their opening hours to cope with the huge demand.
Unlike the first lockdown in March, schools, universities, and nurseries will remain open, and people will be able to meet another person who they do not live with in an outdoor public place such as a park or beach.
The rules says people cannot mix with anyone they do not live with indoors or in private gardens.
New guidance for secondary school pupils and staff
New guidance issued by the Department of Education says adults and children aged 11 and above should wear a mask when moving around the school, outside of classrooms or activity rooms, "where social distancing cannot easily be maintained".
Schools should work to implement the guidance as soon as possible, the department says, but can have until Monday 9 November if they require additional time.
Until now, this requirement was only for schools and colleges where the local Covid-19 alert level was "high" or "very high".
Additionally, staff members with serious underlying health issues should not come into school, the guidance adds.
Paul Whiteman, general secretary of the NAHT school leaders' union said some schools may find it difficult to stay open if they had a number of vulnerable teachers staying at home.
Those most at risk advised to stay at home
Clinically extremely vulnerable people in England are being strongly advised not to go to work outside their homes during lockdown from Thursday.
In updated updated government guidance issued by the Department of Health, those who are at higher risk from Covid-19 are advised to only go out for exercise and to attend health appointments.
People with stage-five chronic kidney disease, those undergoing dialysis and adults with Down's syndrome are now also advised to follow the advice.
Charities supporting these groups, including Asthma UK and the British Lung Foundation, said the guidance was "a step forward" but had given people less than 24 hours to prepare for the changes to their lives. BBC