Seven officers have been injured after police tried to break up an unlicensed music event in west London.
Police were called to White City on Friday evening following reports of a number of people gathered at an estate.
Bricks and other missiles were thrown at police when they tried to speak to the group, forcing them to back away before more officers arrived.
None of the injuries are said to be life-threatening. The violence was “totally unacceptable”, the Met said.
Mayor of London, Sadiq Khan, said: “It’s disgraceful that police officers came under attack with some suffering injuries at an illegal gathering in White City last night.
“Violence against the police will not be tolerated and perpetrators will be caught and prosecuted.”
He added that extra police would be out on Saturday.
Local resident Dave Kayani, who filmed what happened, said when the police initially arrived, they looked like “traffic wardens”.
“They stopped people going into the block and after a while people were getting agitated.”
“The police started to march in, with no riot gear and shouting.
“Some of the people were riled up and started to run towards the police, who started retreating,” he said.
Mr Kayani said riot police arrived about 20 minutes later.
“A bottle hit my leg, not glass. I was okay. It was quite terrifying,” he said, adding that a neighbour had his car window smashed with stones thrown by people.
He said a few illegal parties have been held recently, but this was the biggest.
Police were called by residents complaining about a large gathering, noise, anti-social behaviour and violence.
Officers trained to deal with public disorder were met with hostility and violence, police said.
A Dispersal Zone has been authorised in the area while a section 60 order is also now in place.
Deputy Assistant Commissioner Laurence Taylor said seven officers had suffered injuries as a result of the clashes.
“These gatherings are illegal and also pose a risk to public health.
“The violence shown towards officers this evening was totally unacceptable and we will not tolerate it in any form. Officers encountered bricks and other missiles being thrown at them.”
Police in the capital have been called in to break up a number of illegal gatherings in the past fortnight, with more than 20 officers hurt at a gathering in Brixton, south London, on 24 June.