Beijing: The leader of the student union at Hong Kong's oldest university has fled the city fearing for the safety of his family after being beaten by a masked man.

Hong Kong University's student union president Davin Wong had been a prominent face in the protest movement that has gripped the city over the past three months - declaring a class boycott by students on television a fortnight ago. He had also won the support of the university's vice-chancellor, Zhang Xiang, in opposing police entering the campus.

Hong Kong University's student union president Davin Wong in a Facebook post for his election to the union. Credit:Facebook/DavinWong

But in a letter of resignation sent to the student union council, Wong said Hong Kong had become a "police state" and he had no choice: "By the time [you] read this letter, I have already left Hong Kong... one of the toughest decisions I made and will ever make in my life."

On August 30, within 24-hours of protest organisers Jimmy Sham and Max Chung being beaten by masked men in two separate incidents, Wong was beaten at a bus stop in Wanchai. Another 28 activists and politicians were arrested by police on the same day.