Sir Keir Starmer has vowed that Britain will "never surrender" the flag to "those that use it as a symbol of violence, fear and division" after the Metropolitan Police said dozens of its officers were injured at the Unite the Kingdom rally.
The Prime Minister acknowledged "people have a right to peaceful protest", adding it was "core to our country's values".
However, he added Britain "will not stand for assaults on police officers doing their job or for people feeling intimidated on our streets because of their background or the colour of their skin".
It comes after up to 150,000 people packed central London for the Unite the Kingdom rally, organised by activist Tommy Robinson - whose real name is Stephen Yaxley-Lennon.
The Met confirmed 26 officers were injured, including some with broken teeth, while 25 arrests had been made as a result of "disorder [and] violence" directed at the police.
In his first comments since the protest on Saturday, Sir Keir added that "Britain is a nation proudly built on tolerance, diversity and respect".
"Our flag represents our diverse country," the Prime Minister said.
"And we will never surrender it to those that use it as a symbol of violence, fear and division."
Sir Keir Starmer declared Britain would never surrender the flag to violent protesters
| GETTYDuring the rally, police estimated between 110,000 and 150,000 people attended.
It was addressed by billionaire X owner and ex-Donald Trump adviser, Elon Musk.
Mr Musk told the crowd that "violence is coming" and protesters should "either fight back or you die".
"I really think that there's got to be a change of Government in Britain," he added.
Police clashed with Unite the Kingdom protestors during Saturday's rally | PA"You can’t – we don’t have another four years, or whenever the next election is, it’s too long.
"Something’s got to be done. There’s got to be a dissolution of Parliament and a new vote held."
Sir Keir's comments come as he faces some criticism that he has not done enough to address the protesters.
Labour MPs told the Guardian on Sunday they had been unsettled by comments made by Business Secretary Peter Kyle, which stopped short of condemning the protests.
Mr Kyle said figures such as Mr Robinson were able to "touch into a sense of disquiet and grievance in the community in our society".
He said the demonstrations, like those seen on Saturday, were "klaxon calls to us in public life to redouble our efforts to address the big concerns that people right across our country have, and immigration is a big concern".
However, Labour MP Stella Creasy said on X that "in the spirit of free speech, I disagree with Peter Kyle".
"We can defend right of people to protest and still be concerned that for many watching yesterday purpose behind this event is not freedom but fear," she added.
"They must be able to march and we must challenge message as not who we are."