The Chinese University of Hong Kong, with 20,000 students, is considered the most radical campus. Most of its students are Cantonese-speaking locals, some of whom live nearby with their parents in dense apartment blocks. And the campus is high in the hills of Sha Tin, isolated from the city center, which is an hour’s ride away by subway.

On Monday, the police arrested five students on the campus’s edge, administrators said. The next morning, the police, still at the border, confronted front line students, and clashes took place over 20 hours. Rocky S. Tuan, the president, who has been known for trying to engage with students during the movement, showed up during a lull in the evening to urge the students to be calm.

“You all should know that I really want to help you. I will do everything within my capability,” he said. “It is the university’s responsibility to maintain peace on campus, not the police.”

But as Mr. Tuan began walking away, the police fired tear gas. Mr. Tuan himself was enveloped in the gas. Students set fires to keep the police from advancing, and scores formed human chains to pass along bricks, umbrellas and bottled water to the front lines. Students sitting on one patch of road made gasoline bombs as if on an assembly line.

“It was a savage move and a type of police violence when they tried to encroach on the university,” said Timothy Chow, 23, an engineering student who graduated in June. “This is why we have to protect our Chinese University of Hong Kong.”