Later, President Donald Trump tweeted that U.S. intelligence informed him that Chinese troops were being moved to the Hong Kong border.
The Hong Kong Airport Authority said that airport operations were "seriously disrupted" by a large rally of protesters whom the riot police held back with pepper spray.
Protesters blocked passengers from continuing to immigration checkpoints, and some police cars were blocked during heated encounters with police.
Following scuffles between protesters and police, an injured person was taken away from the main terminal by medics.
Hong Kong's airport had reopened earlier after airport authorities canceled all flights on Monday, blaming demonstrators' disruption to regular operations.
Members of the medical profession gather to protest against Hong Kong police brutality at Queen Elizabeth Hospital on August 13, 2019 in Hong Kong, China.
I hope nobody gets killed," Trump told reporters on Tuesday after he was asked about the Hong Kong protests. »