When students mobilized last year to demand an overhaul of the country’s higher education system and a commitment to free, equal and high-quality public education, the official response was more restrained. This year the government has declared zero tolerance for school occupations, and has called in special police forces to clear the buildings. Hours or days later, the same schools are taken over again, and the police return, a cat-and-mouse pattern that often leads to violent clashes and hundreds of arrests. Meanwhile, small groups of radicalized students set up barricades, throw rocks and damage public and private property.

Protest marches usually erupt in street battles with the police, who use tear gas and chemical-laced water cannons to disperse the crowds and wield their batons to arrest demonstrators. Some students have suffered head injuries, broken noses, convulsions and breathing problems; some have been trampled by police officers on horseback. Increasingly, the observer groups say, detainees are reporting acts of sexual humiliation by the police.

That is why the helmets are there.

Before each protest, they call one another to distribute tasks and locations. On the streets, they wear hard hats marked DDHH — short for derechos humanos, human rights in Spanish — as well as large credential cards around their necks to make their role as clear as possible. They get training in the legal basics, and have strict rules to follow: no interfering in events, no cursing at the police, always work in pairs.

“The first thing we do is approach the officer in charge,” said Ms. Cisterna, a speech therapist. “We tell him we are there to observe police procedures. We don’t intervene, we don’t try to take detainees away from them, but we do let them know when they’re doing something illegal or irregular, that they can’t beat people up, and that we are watching and have their names and ranks. They pay attention.”

Her group was among the first to field observers in white helmets. Members of Sutra, a labor union, also wear them and monitor the police at labor strikes and community protests as well as student actions. A third group created last year, Observers and Defenders of Human Rights, wears blue helmets similar to those of United Nations peacekeepers. A fourth observer group founded last month by law students also provides legal assistance to detainees.