HONG KONG — Chinese border officers have begun routinely searching the phones of people who enter mainland China from Hong Kong, raising concerns that Beijing is trying to identify travelers sympathetic to the territory’s protest movement and further control what its people see about the unrest.

During the phone checks, officers look through photos, messages and other apps, three people whose devices were searched told The New York Times. As far as the travelers could tell, the people selected for extra inspections were mostly young men. The demonstrations have been largely youth-driven.

The searches have come to light as the protests, now in their third month, have grown increasingly violent and disruptive, drawing sharp denunciations from the mainland Chinese leadership and raising the possibility that it might crack down on the demonstrators.

Chinese paramilitary forces have gathered in Shenzhen, across the border from Hong Kong. On Thursday, tarp-covered troop transports and armored personnel carriers were seen parked outside a Shenzhen stadium.