The UK Foreign Office said it was urgently seeking clarity on the incident. Greater Manchester Police has launched an investigation, the BBC reported.

The BBC filmed unidentified men coming out of the consulate and forcing the man inside the compound, before he escaped with the help of police and other demonstrators.

The protester told the BBC "they dragged me inside, they beat me up".

A consulate spokesperson said protesters had displayed an insulting portrait of China's president.

Speaking after the incident, the protester, called Bob, told BBC that "mainlanders" - people from mainland China - 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 police.

"It's ridiculous. They shouldn't have done that. We are supposed to have freedom to say whatever we want here (in the UK)."

Consulate staff had previously asked the protesters to move to the opposite side of the street, the BBC said.

There were two police officers present at the protest, the BBC report added, saying 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.

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. Any offence committed on diplomatic premises is subject to UK law, but employees may hold diplomatic immunity.

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.

The demonstrators were protesting as the Communist Party congress began in Beijing.

Chinese President Xi had said he had turned the situation in Hong Kong from "chaos to governance", referring to China's suppression of pro-democracy protests there.

A spokesperson for the consulate said the protesters had "hung an insulting portrait of the Chinese president at the main entrance".

"This would be intolerable and unacceptable for any diplomatic and consular missions of any country. Therefore, we condemn this deplorable act with strong indignation and firm opposition," the spokesperson added.

A spokesperson told the BBC, Greater Manchester Police was aware of the incident. "Officers were present and responded immediately to diffuse the situation," they said. "Enquiries are ongoing at this time to understand the full circumstances."

(With UNI inputs)