Wearing black clothing, scarves and ski masks, so-called "Black Bloc" anarchists have taken a starring role in Brazil's social unrest, using violent tactics to target symbols of capitalism.
But experts find it hard to build a clear profile of the group, which began attracting notice during June's street protests against substandard public services, corruption and the cost of the 2014 World Cup.
With the social turmoil growing more sporadic, Black Bloc tactics have inspired more violent actions, mainly in Rio and Sao Paulo.
Black-clad anarchists infiltrate protest demonstrations to carry out acts of vandalism against capitalist symbols such as McDonald's restaurants or banks.
On Tuesday, they fought police in Rio de Janeiro and Sao Paulo at the close of a march by striking teachers, hurling firebombs and tearing metal shutters from buildings to use as shields against police who responded with tear gas and pepper spray.
Demonstrators also set fire to a police car and ransacked a bank branch.
A 18-year-old youth was hospitalized with gunshot wounds but was reported to be in stable condition after surgery.
The Black Bloc tactic of using black clothing to conceal protesters' identities and hinder criminal prosecution first appeared in the 1980s in the European autonomist movement's protests against squatter evictions, nuclear power and restrictions on abortion.
But they gained notoriety during the anti-globalization protests that marred the World Trade Organization conference in Seattle in 1999.
"We don't speak to reporters"
In Brazil, they keep a low profile. The Rio branch of Black Bloc turned down an AFP request for an interview, saying: "It is difficult to contact us by telephone because we are wiretapped. We prefer to communicate via Facebook."
"We don't speak to reporters," the group's national office said.
The elusive group is an enigma for experts.
"It is difficult to know who they are. There is no clear hierarchy. The movement is very horizontal, very young," said Luiz Antonio Machado, a sociologist at the Rio Institute of Social and Political Studies (IESP).
"This movement grew out of the general discontent with the poor functioning of institutions. It is not revolutionary but is likely to become more radical due to the absence of dialogue."
Machado believes the establishment media are using the actions of Black Bloc and other rioters to discredit the protest demonstrations, which leads to police repression.
According to Alba Zaluar, who studies violence at the University of Rio, the group "throws away all that our democracy built" after the (1964-1985) military dictatorship.
"This leads to lower attendance at demonstrations. People are not trying to get rid of the state, they just want to improve health, education, transport," she added.
Meanwhile, sociologist Jose Augusto Rodrigues insisted that "not all acts of vandalism should be blamed on the Black Bloc" but he noted that the group's tactic "was not a success."
"In June, there were huge crowds in the streets but since they infiltrated the demonstrations, people are staying away," he added.
Rodrigues said it was difficult to predict the anarchists' future impact on the country's political scene, noting that this will depend on how the state and its police react.
But the Black Bloc has vowed to step up their violent protests during the World Cup, which will kick off next June.
[Image via Agence France-Presse]