The American, Will Nguyen, was visiting Ho Chi Minh City ahead of his graduation this summer from a master’s program at the University of Singapore, according to a statement from his family and friends.

Mr. Nguyen, 32, a Houston native who graduated from Yale, took part in protests on Sunday. He was “beaten over the head and dragged into the back of a police truck,” after the authorities moved to quash the demonstrations that day, according to the statement.

A video from the protests shows Mr. Nguyen, with blood smeared across his face, being dragged by a group of men. He is later shown standing in the bed of a pickup truck topped with emergency lights.

He was taken to a police station, but his current whereabouts and physical condition aredetention. Police officers showed up at the apartment two days later to confiscate his laptop, passport, credit cards and a change of clothes, his family said.

In addition to the special economic zones, protesters said they were concerned about a proposed cybersecurity law. The state-controlled news media in Vietnam reported that 102 people were arrested Sunday in the southeastern province of Binh Thuan, where thousands of protesters blocked a highway and later set fire to public buildings. Protests were also reported in Hanoi, the capital.

The proposed special economic zones would give leases of up to 99 years to foreign investors in three areas that would have fewer administrative restrictions than the rest of the country. The proposal has stirred fear that it would undermine national security by giving China