HONG KONG — The teenage son of a prominent human rights lawyer in China was blocked from leaving the country Monday after the police told him he posed a potential threat to national security while abroad, his father said.

The travel ban against Bao Zhuoxuan, 18, was seen by his family and human rights groups as retaliation against his mother, Wang Yu. Ms. Wang was a commercial lawyer who became involved in politically delicate cases, and was the first person targeted two years ago in a widespread crackdown on human rights lawyers in China.

Mr. Bao had been planning to travel in Japan for two weeks to celebrate having successfully taken a test to study abroad, according to his father, Bao Longjun. The family had recently been allowed to return to Beijing after spending two years under strict surveillance in the Chinese autonomous region of Inner Mongolia.

The authorities had returned the teenager’s passport, and he was able to obtain a visa to go to Japan. But on Monday he was stopped and held while passing through immigration at the Tianjin airport. The corners of his passport were also cut, rendering it invalid.