A Hong Kong pro-democracy protester was pulled into Chinese consulate grounds in Manchester and beaten up, media reports said.
Unidentified men came out of the consulate and forced a man inside the compound before he escaped with the help of police and other demonstrators, BBC reported.
The protester told the BBC: "They dragged me inside, they beat me up."
The UK government called the reports "extremely concerning". The consulate said the protesters displayed an insulting portrait of Chinese President Xi Jinping.
The Foreign Office said it is urgently seeking clarity on the incident. Greater Manchester Police has launched an investigation.
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Speaking after the incident, the protester, named Bob, told BBC Chinese that "mainlanders" - people from mainland China, as opposed to Hong Kong - had come out of the consulate and destroyed their posters.
As we tried to stop them, they dragged me inside, they beat me up," he said, adding that he was then pulled out by the UK police.
"It's ridiculous. They [the attackers] shouldn't have done this. We are supposed to have freedom to say whatever we want here [in the UK]."
After the incident, the crowd remained angry. The protesters shouted at the men from the consulate and the British police, arguing that they could have done more.
There were two police officers at the protest, but several more appeared within minutes of the altercation beginning.
They gathered at the gates of the compound trying to break up the fighting and move protesters back.
One police officer entered the consulate grounds and pulled the man who had been dragged inside back out.
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At least eight men - some of whom were wearing helmets and protective vests - then returned to the consulate building.
The consulate is on UK soil, but cannot be entered without consent.
Reacting on Twitter, former Conservative leader Iain Duncan Smith said the UK government should demand a full apology from the Chinese ambassador, and that those involved should be sent back to China, BBC reported.
