The UK will officially leave the European Union at 23:00 GMT, ending 47 years of membership.
In a video message to be released an hour earlier, Prime Minister Boris Johnson – who led the 2016 campaign to leave – will call Brexit a “new dawn”.
Pro and anti-Brexit demonstrations and marches are being held across the country, as the UK flag is taken down from EU institutions in Brussels.
Little will change immediately, as the UK begins a “transition period”.
Most EU laws will continue to be in force – including the free movement of people – until the end of December, by which time the UK aims to have reached a permanent free trade agreement with the EU.