Hong Kong’s secretary for justice Teresa Cheng suffered an injury to her arm during the incident on Thursday.
In a statement, the Met Police said: “A woman was taken to hospital by London Ambulance Service suffering an injury to her arm.
“Enquiries are ongoing to establish the full circumstances. There has been no arrest at this stage.”
The alleged assault follows the death of 22-year-old Alex Chow Tsz-Lok, who died last week in Hong Kong after falling from a car park when police fired tear gas at protesters.
The Chinese and Hong Kong governments have condemned the “attack” on Ms Cheng.
A statement from the Hong Kong government said the official suffered “serious bodily harm”, but gave no further details.
Video footage of the incident shows the minister surrounded by protesters and falling to the ground.
Hong Kong’s leader Carrie Lam strongly condemned the “attack”, saying the incident was “barbaric” and transcended the bottom line of a civilised society.
“The secretary denounces all forms of violence and radicalism depriving others’ legitimate rights in the pretext of pursuing their political ideals, which would never be in the interest of Hong Kong and any civilised society,” the Hong Kong government said in a separate statement.
The Chinese embassy in London said Ms Cheng was pushed to the ground and sustained a hand injury.
“(Cheng) was besieged and attacked by dozens of anti-China and pro-independence activists,” the Chinese embassy said in a statement.
The incident showed that the “violent and lawless perpetrators” were now taking their violence abroad, it said.
China has lodged a formal complaint with Britain and urged British authorities to bring the perpetrators to justice.
A demonstrator who asked to remain anonymous told PA that the incident had been exaggerated.
“Hong Kongers of course took the opportunity to protest,” they said.
“It was small scale and people were chasing after her and yelling at her, and amid chaos, she did trip over.
“Now, the Hong Kong government is calling the protesters “violent mob” and “radicals”, reporting the case to Met Police, making it pretty big.”
Ms Cheng is a chief legal adviser to Carrie Lam and was responsible for advising the Hong Kong leader on the controversial extradition bill.
In Hong Kong, a 70-year-old man has died after being struck on the head during clashes between government supporters and protesters.
Hong Kong’s Food and Environmental Hygiene Department expressed “profound sadness” at the death of its cleaning worker and said it was providing assistance to his family.
Authorities in Hong Kong are now treating the death as a murder.
Meanwhile, anti-government protesters have again paralysed parts of Hong Kong, forcing schools to close and blocking some roads as students built barricades in university campuses.
In a 3am statement, a masked protester at the Chinese University of Hong Kong announced a road being partially blocked by protesters would be cleared for 24 hours.
Workers sent to clean up a lane of the Tolo Highway were threatened by protesters with bows and arrows and hard objects, authorities said.
The protests escalated in June over an extradition bill that would have allowed people to be sent to mainland China for trial.
The bill was later scrapped but the protests have since evolved into calls for greater democracy, among other demands.
The demonstrations have plunged the former British colony into its biggest political crisis in decades and pose the biggest challenge to Chinese President Xi Jinping since he came to power in 2012.