Photo/Illutration In this photo released by Xinhua News Agency, Chinese Communist Party leaders, including from left Zhao Leji,, Wang Yang, Premier Li Keqiang, President Xi Jinping, Former President Hu Jintao, Li Zhanshu and Wang Huning, attend the opening ceremony of the 20th National Congress of the Communist Party of China at the Great Hall of the People in Beijing on Oct. 16. (Xinhua via AP)

LONDON--British police said Monday they were investigating the assault of a Hong Kong pro-democracy protester who was beaten up on the grounds of the Chinese consulate in the city of Manchester.

The man, who is in his 30s, told the BBC that unidentified Chinese men from the consulate building dragged him inside the consulate and assaulted him during a protest Sunday.

Greater Manchester Police said its officers intervened and removed the protester from the consulate grounds “due to our fears for the safety of the man.”

“Around 30 to 40 people gathered outside the Chinese Consulate, and local patrols were in the area to help maintain the peaceful protest,” police said. “Shortly before 4 p.m., a small group of men came out of the building and a man was dragged into the consulate grounds and assaulted.”

The protester remained in the hospital Monday for treatment, police said. No arrests have yet been made.

Video on the BBC website showed a scuffle breaking out in front of the consulate after masked men tore down and took away the protesters’ placards. The video appeared to show several men wearing face masks beating up someone who had been pushed to the ground amid the scuffles.

Downing Street said the incident was “deeply concerning,” adding that police had stepped up patrols in the area.

The Chinese consulate in Manchester did not immediately respond to requests for comment. Chinese Foreign Ministry spokesperson Wang Wenbin told reporters in Beijing that he was not aware of the incident but added the Chinese embassy and consulates in the United Kingdom always abide by local laws.

At a press briefing Tuesday, Hong Kong leader John Lee said he did not have the full details about the case but trusted that the U.K. government would handle the incident in accordance with the Vienna Convention on Consular Relations and British law.

Andy Burnham, the mayor of Greater Manchester, said it was important for the full facts to be established, but added: “On the basis of what I have seen, I want to make clear that it is never acceptable for peaceful protesters to be assaulted and those responsible need to be held to account for their actions.”

The opposition Labor Party’s foreign affairs spokesperson, David Lammy, called on the Conservative government to summon the Chinese ambassador to explain what happened.

The protesters had gathered as the Chinese Communist Party opened its weeklong congress in China on Sunday.