Photos posted on social media showed Mr Chung being led away in handcuffs in the district of Yuen Long.
Prominent rights activist Joshua Wong said Mr Chung had been followed by police for several days.
He said Mr Chung had been arrested for writing a Facebook post on "China's nationalism" and alleged
that the detainees' phones had been hacked shortly after their arrest.
A new road to a high-altitude Indian forward air base is said to have been one of the main triggers
for a clash with Chinese troops last month that left at least 20 Indian soldiers dead.
The 255km (140-mile) Darbuk-Shyok-Daulat Beg Oldi (DSDBO) road - which winds through mountain passes
up to the world's highest airstrip more than 5,000m above sea level in the Ladakh region - was finished
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