The demonstrators accuse police of causing the injury by firing a bean-bag round, and cite the case as evidence of what they say has been an excessive and disproportionate response by police to their protests.
The activists have called on their supporters to return to the airport later on Tuesday, though it was unclear whether authorities would allow that to happen.
The protests that began in opposition to a bill allowing extradition to the mainland has morphed into a broader bid to reverse a slide of freedoms in the southern Chinese city.
But the city’s Beijing-backed leader has ruled out meeting the protesters’ demands, which include the right to choose their next leader and an investigation into alleged police brutality.
The demonstrations have become increasingly violent, with police using tear gas and rubber bullets to push back protesters who have sometimes hurled bricks and bottles.
“It is becoming more and more dangerous, but if we don’t still come out at this point, our future will become more frightening, and we will lose our freedoms,” a 22-year-old protester at the airport told AFP on Monday.