HONG KONG >> High school students in Hong Kong starting the new school year Monday arrived to class wearing gas masks and joined hands to form human chains as riot police officers took up positions around schools. University undergraduates held a strike, waved flags and chanted protest slogans.

After a summer of demonstrating in the streets, outside municipal offices and in the airport, students refuted the government’s wishful assertion that once they returned to school the months of pro-democracy protests that have roiled the city would come to an end.

“The government thinks it can quell the movement when students return to school, because we can only come out during the summer,” said Owen Lo, 16, a high school student. “But that’s not true.”

He said he was afraid of the repercussions he and other students might face, but “seeing so many students selflessly gambling their future to express their demands to the government, it is infectious, and makes me want to come out and do something for Hong Kong.”

Students who gathered at a plaza near the Central Business District were joined by thousands of workers holding a concurrent strike, exemplifying the multigenerational makeup of the protests.

“Through these peaceful activities, many citizens are coming out to show their comrades on the front lines that public opinion has not turned against them,” said Peter Chui, a 24-year-old engineer.

Hundreds of thousands of people have joined protests that began in June to oppose an unpopular bill that would have allowed Hong Kong residents to be extradited to mainland China for trial. As the summer unfolded, the protesters’ demands grew to include a call for universal suffrage and an independent inquiry into accusations of police brutality. The protests have also become more violent, as a core group of demonstrators — many of them high school and university students — have fought with the police and damaged symbols of Chinese authority.

A senior Hong Kong official on Monday warned for the first time that “elements of terror” were seen among the city’s pro-democracy protesters after a weekend of demonstrations punctuated by violence and vandalism.

The Hong Kong authorities had previously rejected a mainland official’s description of “signs of terrorism” in characterizing acts of violence by some protesters. But John Lee, the territory’s secretary for security, changed that stance Monday, as the local government escalated its criticism of the monthslong demonstrations.

“The extent of violence, danger and destruction have reached very serious conditions,” Lee said. “Radical people have escalated their violent and illegal acts, showing elements of terror.”

Police officers attacked unarmed demonstrators inside a subway station Saturday, and protesters Sunday disrupted transit for thousands of travelers at Hong Kong’s international airport, the seventh-busiest in the world.

Tensions on Monday, the first day of school for many students, ran high. Police officers monitored the student protests and riot police officers were deployed to subway stations — a sign of the fears that the weekend’s tumultuous protests would continue onto campuses. But the activism Monday morning remained peaceful.

High school students around the territory protested in various ways across campuses, reflecting the diversity of the movement. They wore black shirts or eye patches to commemorate a first-aid volunteer who recently lost an eye to a projectile shot by the police. Others studied in the library or designated classrooms rather than participate in normal classes.

A group of secondary school students from St. Francis’ Canossian College, the alma mater of the city’s leader, Chief Executive Carrie Lam, carried signs and donned construction helmets or gas masks, gear typically saved for more violent protests. One sign read: “Chief Executive, will you listen to the voices of the younger sisters from your school?”

Tens of thousands of students from schools and universities across the city crammed the central commons on the campus of the Chinese University of Hong Kong to listen to speeches from professors, lawyers and student protesters.

Education and government officials said they were opposed to class boycotts in any form, saying in a letter to teachers that schools should remain politically neutral places.

“Schools are absolutely not places for presenting political views or demands,” Matthew Cheung, the city’s chief secretary, said at a news conference Monday.

Boycotts, he said, would disrupt normal classes, “extending the turmoil we see in society to the peaceful environments of school.” He added, “This holds no benefits for the mood of students as they study, or for the healthy growth of our next generation.”

When asked why police had showed up at high schools, the education secretary, Kevin Yeung, said protests held by “unknown people” outside schools could not be treated like boycotts on campuses.

“The schools clarified that they did not call the police,” he said. “When people see police in a place with a lot of people, they want to maintain order and ensure public safety.” He said officers should not be seen as a threat.

Joshua Wong, a prominent 22-year-old activist whose activism as a secondary student played a prominent role in citywide protests, said that class boycotts represented the most peaceful forms of resistance against Beijing.

“When we can’t see the future of our society, how can we see our personal future and our personal career?” he said in a phone interview. The class boycotts were not the “starting point and not the end point” of the protest movement, he added. “It just shows how our momentum keeps going forward.”