People across England took advantage of new rules allowing pubs and restaurants to serve drinks and food outside.
Many people braved cold temperatures and raised a glass just after midnight when pubs were allowed to open.
The relaxing of measures was welcomed by the hard-hit hospitality industry after repeated closures.
Prime Minister Boris Johnson called the easing of restrictions "a major step forward in our roadmap to freedom."
"I'm sure it will be a huge relief for those business owners who have been closed for so long, and for everyone else it's a chance to get back to doing some of the things we love and have missed," he said, adding that he as well planned to toast the latest easing of COVID rules with a celebratory pint at a pub beer garden.
What else is reopened in the UK?
In England and Wales, nonessential shops and beauty salons can now resume business with health and hygiene measures in place.
Gyms, zoos and hairdressers in England can also reopen. Long lines have already formed outside shops around the country.
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Shoppers were seen queuing outside fast-fashion retail chain Primark in Birmingham for its 7 a.m. reopening this morning.
Students in Scotland and Wales are returning to school full-time, while Northern Ireland's "stay-at-home" order is ending.
Mosques, meanwhile, are preparing for the start of Ramadan this week, following a year of canceled traditional community and family gatherings.
Secretary general of the Muslim Council of Britain, Zara Mohammed, said the UK's resumption of Ramadan prayers could give worshippers "renewed optimism."
How have small businesses handled the pandemic?
But Monday's reopening came too late for some businesses. Months of lockdowns forced hundreds of smaller businesses to close down altogether — in particular local English village pubs, a popular gathering spot for community members.
About 2,000 pubs shut their doors for good during the pandemic, causing many employees losing their jobs, the British Beer and Pub Association estimated in March.
Johnson warned people must "behave responsibly" and continue to adhere to health measures to "suppress COVID."