Former student activist Nathan Law
, 24, was pushed and attacked by protesters holding signs reading "destroy Hong Kong independence" in the arrival hall of Hong Kong International Airport.
He was struck, his shirt ripped and liquid thrown over him, according to police, causing injuries to his neck and shoulder.
Hong Kong police said the assault is under investigation, but no arrest has yet been made. Following the incident 25 pro-democracy lawmakers issued a statement condemning "this violent act that flies under a so-called flag of patriotism."
Law was returning from a political forum in Taiwan which he had attended with several other Hong Kong pro-democracy figures, including Joshua Wong and two other lawmakers.
Both Law and Wong were heavily involved in the 2014 Umbrella Movement which saw tens of thousands of pro-democracy protesters shut down key streets in Hong Kong for months.
They were pushing for free elections to choose the city's leader. The occupation ended when police moved in and cleared the site, and years later none of the original aims of the protest leaders have been achieved.
On the way to the Taiwan event, Law and Wong said they were followed by a group of dozens of protesters at Hong Kong airport, who held signs calling them "traitors" who "collaborate with Taiwan independence forces."
As they arrived, protesters at Taipei's Taoyuan International Airport also screamed insults and attempted to assault Wong. Taipei Mayor Ko Wen-je denounced the violence in a statement.
According to Taiwanese state media
, the protesters were from the Patriot Association, which opposes Taiwan independence and advocates for unification with China.
Ma Ngok, a professor at the Chinese University of Hong Kong, said that such harassment of activists and politicians is a common tactic of pro-Beijing groups.
"Authorities are very concerned about the collaboration of Hong Kong and Taiwan (politicians)," Ma said.
He said the twin airport protests indicated such protests were occurring at a "bigger and more coordinated level."
"It's more complicated than it used to be," Ma said.
Beijing is sensitive to any calls for independence in Hong Kong and Taiwan, which is self-governing but is claimed by China under the "one China" policy.
In December, President-elect Donald Trump called into question the future of the policy by taking a congratulatory phone call from Taiwan's President Tsai Ing-wen.
The policy, enshrined in the documents that eventually led to the establishment of US relations with China, permits Beijing to regard Taiwan as a part of China and the United States to sell the nationalist island arms to defend itself against the mainland, and has headed off a major US-China clash over the issue.