CLASS warriors striving to upend Brazil’s stratified society? Disaffected youth upset at the lack of opportunities? Black favela-dwellers tired of the country’s veiled racism? Theories abound over the motives of the participants of rolezihnos, gatherings organised via social networks in which tens, hundreds, and sometimes thousands of youngsters from São Paulo’s poor periphery meet up in the city’s shopping malls. What do they really want? “To hang out, chill, meet people—especially girls,” is the answer of Rodrigo Alexandre, a distinctly light-skinned 17-year-old from Vila Cachoeira, a northern suburb of the city. Together with a few friends Rodrigo had called a rolezihno (dubbed “flash mobs” by Western media) for this weekend at Shopping Center Norte, a 20-minute metro ride from city centre. Roughly 1,300 people confirmed on Facebook that they would attend at 4pm on Saturday January 18th.

Before the revelries could begin, however, two black police SUVs pulled up alongside Rodrigo and nine others as they strolled through Center Norte’s vast car park. The boys were apparently identified on the basis of Facebook photos. Four burly policemen rounded them up, took down their names and held them for an hour to ensure they didn’t make trouble. A gaggle of thirty-odd journalists, including two camera crews, flocked to the scene. Like your correspondent, the police had also seen the Facebook page and were staking out the mall. An hour later the boys were free to go, though not before sharing detailed accounts of their predicament with the press.

Rolezinhos have been taking place in São Paulo in one form or another since October. They gained notoriety on January 11th, when the police used rubber bullets and tear gas to disperse a rowdy crowd of several thousand youths in Shopping Metrô Itaquera, in the east of the city. Geraldo Alckmin, governor of São Paulo state, has ordered an inquiry into whether the police overreacted. Several swanky malls in the city centre subsequently secured court orders to bar suspicious-looking youths from entry.

Mall-owners and shopkeepers have reason to be wary. It does not take much for a band of a thousand adolescents to go from making merry to causing mayhem. A few rolezinhos have led to muggings and robberies, prompting police action, although most do not end in Itaquera-like chaos. Two shopping centres in Rio de Janeiro remained closed this Sunday to forestall planned rolezinhos; the 50 or so participants who turned up listened to funk and set up a grill on the pavement outside instead.

Yet in São Paulo, at least, fancy shopping arcades in the centre have less to fear than those nearer the periphery. As Evandro Farias de Almeida, a Facebook fashion idol to rolezinho youth told Veja, a local weekly: “Why would I spend two hours on a bus just to go shopping where everything is more expensive and nobody recognises me?” The whole point of a rolezinho is to see and be seen.

If that sounds innocuous, the authorities are nonetheless spooked. The memory of demonstrations in São Paulo against a proposed rise in bus fares, which morphed into the biggest nationwide protests in 20 years and brought millions of Brazilians onto the streets in 100 cities last June, remains fresh. In several cities, including the capital, Brasília, social movements from anti-racism activists to Black Blocs, a shadowy group born of last June’s protests and prone to vandalism, have called pseudo-rolezinhos. More are planned for the coming weeks as different groups appropriate the phenomenon for their own ends. Even so, police reactions such as that in Center Norte’s car park look like overkill. “It’s discrimination,” Rodrigo grumbles. He did not arrive at Center Norte with a political agenda. He may have left with one.