Police are warning people not to attend New Year celebrations on the River Thames in London without a ticket, as spectators gather to usher in 2020.
More than 100,000 tickets have been bought for the sold-out fireworks display.
Firework shows are also to be held in cities including Manchester, Cardiff, Newcastle, Inverness and Nottingham.
Hogmanay celebrations have begun in Edinburgh, where crowds have gathered ahead of a massive display at midnight.
Fireworks have already been let off from Edinburgh Castle.
The Metropolitan Police urged those without tickets in London to watch from home or attend other events in the city.
Ticket-holders who had queued at designated entrances in London rushed to pick out their spots when the gates opened.
In a statement to those visiting London for the celebrations, the Met said it wanted “everyone who comes to London for New Year’s Eve to have a good time”.
However, referencing the fireworks on the Thames, the force added: “If visitors do not have a ticket, entry will not be permitted to the event, so the advice from the Met is to watch the fireworks from the comfort of your home.”
In London, approximately 12,000 fireworks are set to light up the capital’s skyline when the clock strikes midnight on Tuesday. Roughly 2,000 of the fireworks will be fired from the London Eye, with the remainder coming from barges that will be moored in a central location along the River Thames.
Big Ben’s chimes will sound the start of the display, despite them being silent this year while renovation work is completed.
Mayor of London Sadiq Khan said the the city’s role in hosting several key games in the Euro 2020 football championship would be celebrated.
“We may be leaving the European Union, but we’re not leaving Europe. So tonight’s fireworks celebrate us as a global city, us as a European city,” he added.
He stressed that London and the UK need to be brought “together again” in the 2020s.
“I’m not pretending that fireworks and one night can do that, but I think it’s really important [that] we celebrate, tonight, some great things about our city and our country,” he said.
In Edinburgh on Monday night, crowds created a huge “be together” symbol of two people reaching out a hand in friendship in a display of fire art in Holyrood Park.
On New Year’s Eve itself, tens of thousands of people are expected to attend the Hogmanay street party featuring acts including Mark Ronson, Marc Almond and Idlewild.
The event covers at least part of more than a dozen streets in the city centre.
The evening is set to be cold but dry for many, with temperatures in Scotland and parts of northern England forecast to be around 1C (33.8F) or 2C (35.6F), according to the Met Office.
The rest of the UK is likely to see the mercury fall to around 5C (41F).
A yellow weather warning for fog has been issued by the Met Office for parts of north eastern England. The warning is in place from 19:00 GMT on New Year’s Eve until 03:00 GMT on New Year’s Day.
Parts of central England could see some drizzle on Tuesday evening while there may also be patches of fog appearing across the UK, according to Met Office forecaster Greg Dewhurst.
However, it is unlikely that visibility will be poor in either London or Edinburgh, he added.
New Year celebrations have already begun in some parts of the world.
The first places to welcome 2020 included the tiny Pacific island of Kiribati, neighbouring parts of Samoa and the Chatham Islands.
Auckland in New Zealand was the first major city to ring in the new decade, with thousands welcoming 2020 at a fireworks display at the city’s Sky Tower.
The traditional fireworks display in Sydney Harbour also went ahead, despite calls for it to be cancelled due to Australia’s bushfire crisis.
The uninhabited Baker Island and Howland Island, on the other side of the International Date Line, will be the last to leave 2019 behind.