Hong Kong's leader Carrie Lam on Friday condemned a "barbaric attack" on her justice minister, who fell while being surrounded by a crowd of jeering pro-democracy protesters in London.

It was the most physical confrontation involving a member of Lam's cabinet since the protests, now in their sixth month, erupted in the international finance hub.

Teresa Cheng, Hong Kong's deeply unpopular Secretary for Justice, was ambushed by around a dozen masked demonstrators as she prepared to attend a speaking event on Thursday night in London.

They shone torch lights on her while calling her a "murderer" and shouting slogans of the Hong Kong pro-democracy protest movement.

Video of the incident showed Cheng falling to the floor during the melee -- although it was not immediately clear if she was pushed by protesters.

She regained her feet moments later and was escorted away with no visible signs of injury.

Cheng suffered "serious bodily harm" during the altercation, Chief Executive Lam said in a statement.

Cheng, whose department is in charge of prosecuting protesters who have filled city streets for months, is the first senior Hong Kong official to be injured in scuffles since the unrest began in June.

Lam called on police in Britain -- Hong Kong's former colonial ruler -- to investigate, describing the incident as an "attack" which "was barbaric and violated the principles of a civilised society".

Cheng is in London on a visit to promote Hong Kong’s role as a dispute resolution and deal-making hub.

She is one of the most unpopular government officials in Hong Kong, seen as playing a key role in pushing forward the now-shelved extradition bill to China, which sparked the ongoing unrest.

The protests have tipped into worsening violence with two dead in a week and further protests planned for Friday.

For a fifth straight day on Friday, protesters caused widespread disruption with barricades and rallies.

Overstretched police have drafted in reinforcements, while Hong Kong's government has denied rumours of an imminent curfew.

The five-month crisis has entered a new phase in recent days with hardcore protesters embarking on a campaign to "blossom everywhere" across the city in a bid to stretch police resources.

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