Prime Minister Boris Johnson has urged caution as most legal restrictions on social contact are lifted in England.
There are now no limits on how many people can meet or attend events, nightclubs reopened at midnight, and table service will not be necessary in pubs and restaurants.
Face coverings will be recommended in some spaces, but not required by law.
Vaccines Minister Nadhim Zahawi said government experts had recommended that only some under-18s be vaccinated.
The PM, chancellor and the health secretary are self-isolating, and there are warnings cases will surge.
On BBC Radio 4's Today programme, Mr Zahawi defended the U-turn which saw the prime minister and chancellor self-isolating, after initially saying they would take part in a pilot programme to take daily tests instead.
Mr Zahawi insisted that Mr Johnson only "considered" joining the pilot scheme, adding: "No one is taking anyone for fools."
Some scientists are predicting that UK infections - currently at about 50,000 a day - could reach 200,000 a day later in the summer.
But with more than 68% of UK adults fully vaccinated modelling suggests hospital admissions, serious illness and deaths from Covid-19 will be at a lower level than in earlier peaks.
In a video posted on Twitter on Sunday afternoon, the prime minister said now was the "right moment" to move to the final stage of England's roadmap out of lockdown.
"If we don't do it now we've got to ask ourselves, when will we ever do it?" he asked, adding that the virus would have the "advantage of the cold weather" in autumn and winter.
"But we've got to do it cautiously. We've got to remember that this virus is sadly still out there. Cases are rising, we can see the extreme contagiousness of the Delta variant."
Labour's shadow health secretary Jonathan Ashworth said ending legal mask requirements and the recommendation to work from home was "reckless" and warned of a "day of chaos" on the transport network as people returned to offices after months away.
"The last thing we want on a day like this is chaos and for the public health measures to be undermined," he told BBC Radio 4's Today programme.
But Nadhim Zahawi defended the reopening, saying 90% of the most vulnerable had been vaccinated and it was right for people to take "personal and corporate responsibility" over measures such as wearing masks.
"We're doing the right thing to get as close to normal as possible, as quickly as possible," he told BBC Breakfast, saying that the end of the school term would reduce the spread of the virus.
He also said he had received the recommendation from the Joint Committee on Vaccination and Immunisation about vaccinating children aged 12 to 17 and would make a statement to Parliament on Monday afternoon.
The committee has recommended giving the vaccine to children with conditions that make them vulnerable to Covid, children who live with someone vulnerable and those who are close to turning 18, he said.
As restrictions ease in England, businesses such as nightclubs - and large events - will be encouraged to use the NHS Covid Pass to check people are fully vaccinated, have natural immunity from infection or have a recent negative test.
At Fibre in Leeds, these checks were not being used. While some clubgoers said they were a little nervous, most were overjoyed to return to the dance floor.
"It's been literally the best night of our lives. We've been waiting for this since we turned 18. This has been like life has come back to normal all of a sudden," said Mollie, 20.
Mark, an estate agent, said he had previously had Covid and was now fully vaccinated. "I want my life back," he said.
Terry George, the owner of Fibre, said many of his customers told him they had not been vaccinated or only had one dose so they would not get in if proof of full vaccination was required. But he also said the government had not provided the tools to scan for vaccination status.
Contact tracing and self-isolation requirements will remain in place as other restrictions end.
UK-wide guidance recommending against travel to amber list countries is also being removed. Adults who have been fully vaccinated in the UK will not have to self-isolate for 10 days after visiting these countries - with the exception of those returning from France to England, Scotland or Wales, because of concerns over the spread of the Beta variant of the virus.