More than 1.4 million people in South Yorkshire are the latest to move to England's top level of restrictions.
Tier three measures came into effect at midnight affecting areas including Doncaster, Rotherham and Sheffield.
Sheffield City Region's mayor said the measures were needed but called on the government to "define precisely what the exit criteria is" from tier three.
Meanwhile, Wales entered the first full day of a national lockdown amid border patrols to stop non-essential travel.
Gloucestershire Constabulary said it will patrol routes into the Forest of Dean area and pull over vehicles suspected of making unnecessary journeys out of Wales.
Drivers without a valid excuse will be advised to turn around and, if they do not, will be reported to police in Wales who can issue fines, the force added.
It comes as another 174 deaths and 23,012 new confirmed cases were recorded on Saturday.
And a leading epidemiologist has warned sending some children home from schools may be the only way to control infection rates.
Prof Neil Ferguson, a former government scientific adviser, told BBC Radio 4's Today programme that the current restrictions on household mixing "should have a significant effect".
But he said beyond that there was a "limit to what we can do" without sending some school year groups home.
Prof Ferguson, whose advice led to the lockdown in March, also said it was "too early to say" what impact the restrictions were having, adding: "I think we'll have to wait another week or two."
Asked what the impact would be of relaxing lockdown rules for one or two days on Christmas, Mr Ferguson, who quit his role in May after an "error of judgement", said it was a "balancing act" and "a political judgement".
"It risks some transmission and there will be consequences of that," he said. "But if it's only one or two days the impact is likely to be limited."
Some 7.3 million people are now living under England's tightest restrictions.
As the Sheffield City region entered tier three - very high alert - mayor Dan Jarvis, urged people to "do their bit" and stick to the new rules.
He told BBC Radio 4's Today programme that he was "clear what it takes our end" to get out of tier three, such as a drop in new cases - but the government "do have to be clear and transparent about the exit strategy".
Elsewhere, Stoke-on-Trent, Slough and Coventry moved into tier two - high alert level - at midnight.
In Wales, a 17-day "firebreak" has started, meaning most non-essential businesses are closed, with people only able to leave home for limited reasons.
In line with new guidance, supermarkets removed non-essential items from sale - including clothing, kitchen electrical items and crockery - using barriers and plastic sheets to cover products.
In Scotland, a five-level system will be introduced from 2 November. The top level would be close to a full lockdown, but the aim is for schools to remain open at all levels.
In Northern Ireland, schools have been closed for two weeks as part of an extended half-term break. This is part of a four-week "circuit-breaker" lockdown, with some businesses being told to close temporarily.
Under England's tier three rules, pubs and bars not serving substantial meals have to close, while household mixing is banned indoors and outdoors in hospitality settings and private gardens.
Additional rules in South Yorkshire include the closure of betting shops, adult gaming centres, casinos, and soft play centres. However, gyms will remain open.
The new measures will be reviewed after 28 days, but Sheffield's director of public health, Greg Fell, said he feared four weeks "will not be long enough".
In a letter to residents, Mr Jarvis, who is also the Labour MP for Barnsley Central, said there was light at the end of the tunnel and the restrictions would "help us reach it sooner, and at a lower cost".
He warned South Yorkshire communities now have some of the highest numbers of cases in the north of England and infection rates are still going up.
In Barnsley the infection rate in the seven days to 19 October was 486 cases per 100,000 people, in Sheffield 415, in Rotherham 407 and in Doncaster 393. The average area in England had 117.
Mr Jarvis wrote: "It's tempting to think that because new restrictions are not a silver bullet they are not worth the disruption.
"We don't have the luxury of easy choices. But I have no doubt this was the right one to make.
"The alternatives carry far too great a risk of causing more deaths, and ultimately more harm to our economy."
South Yorkshire joins Greater Manchester, Liverpool City Region and Lancashire in tier three.
Tier three rules will also come into force in Warrington on Tuesday, two days earlier than initially planned, to "urgently" reduce coronavirus cases, according to the local council.
Nottingham and parts of Nottinghamshire are expected to be moved into the highest tier next week, with the finer details such as whether or not gyms can stay open still to be decided.
Meanwhile, Office for National Statistics data estimates cases in England have risen to more than 35,200 a day.
The ONS survey tests a representative sample of the general population to provide an estimate of the true spread of the virus, as it picks up asymptomatic cases that would not necessarily be identified in the daily figures.
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