Police in London said they arrested five people on Saturday and eight officers were injured amid disorder at large-scale protests against England's remaining coronavirus restrictions.
The event was billed "Unite for Freedom" by the organizers.
The protesters see COVID-19 restrictions as unnecessary and a breach of their human rights, including the mandatory use of masks and possible introduction of so-called vaccine passports.
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Demonstrators had gathered in the early afternoon and marched along several major arteries, including the main shopping district Oxford Street, with videos and photos showing thousands in attendance.
Hundreds then rallied in Hyde Park later following the march, an AFP news agency reporter said, where "small pockets of disorder" broke out, according to London's Metropolitan Police Service.
"Eight officers were injured as they worked to disperse crowds in Hyde Park this evening," it said, adding that missiles including bottles were thrown.
"Two officers were taken to hospital. Thankfully, they are not believed to be seriously injured. Five people were arrested for offences including assault on police and are now in custody."
Organizers used various websites and online platform pages to encourage turnout to protest the pandemic measures. /Toby Melville/Reuters
England began to gradually lift COVID-19 restrictions in March after months of restrictions and a successful mass vaccination campaign. Pubs were allowed to serve customers outdoors this month and non-essential shops reopened, but some curbs remain.
Organizers used various websites and online platforms to encourage turnout to protest the pandemic measures, despite rules, one of which limits outdoor gatherings to 30 people. An exception for protests carries requirements including a COVID-19 risk assessment being conducted.
"Anyone coming into London for a protest must make sure their gathering is lawful, with a risk assessment carried out by the organizer," the Metropolitan Police had warned on Twitter.
The government is considering introducing a vaccine passport scheme for a variety of settings. /Toby Melville/Reuters
England's latest COVID-19 lockdown measures have been in place since early January, after the UK saw a surge in cases, and have been eased in recent weeks.
Another major easing is due in mid-May, when some indoor socializing can resume, before nearly all restrictions are ended in late June – if the latest infection rates allow.
The government is looking at introducing a passport scheme for a variety of settings, including leisure and hospitality as well as foreign travel, to verify if people have been vaccinated or have immunity from the virus.
Video Editing: Sam Cordell