Boris Johnson has condemned as "disgraceful" footage of a journalist being abused by protesters.
A clip shared on social media shows demonstrators haranguing BBC Newsnight's political editor Nicholas Watt on Monday outside Downing Street.
A crowd had gathered to protest the extension of coronavirus restrictions for a further four weeks.
The protesters shouted "traitor" at Mr Watt and accused him of "lying" about lockdowns being legal.
The prime minister tweeted: "Disgraceful to see the hounding of Nick Watt doing his job.
"The media must be able to report the facts without fear or favour – they are the lifeblood of our democracy."
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In a statement, BBC director-general Tim Davie said: "The safety of journalists is fundamental to any democracy – they must be able to report unhindered free from abuse.
"There is absolutely no justification for any journalist to be treated in this way."
The footage was also condemned by the Home Secretary, Priti Patel, while media minister John Whittingdale said the incident was "totally unacceptable" adding that the UK was "taking the lead" in tackling intimidation of journalists through its national action plan.
Measures in the plan include new training for police officers and journalists, as well as "commitments from social media platforms and prosecution services to take tough action against abusers".
Labour's shadow culture secretary Jo Stevens, said: "This extremely disturbing footage showing clear intimidation of a journalist while carrying out his job is absolutely unacceptable and should be condemned in the strongest terms.
"It is shocking that a BBC lanyard makes someone a target like this."
The Metropolitan Police said it was investigating the incident adding "anyone with information is asked to call 101 or Crimestoppers on 0800 555 111."
Following suggestions online that officers should have intervened in the incident, the force said: "In this instance, while officers were nearby as part of the policing response to the ongoing protest, they were not in the immediate vicinity of the incident."