Botellones seem to be a phenomenon that occur mainly in Spain, in which teens between ages thirteen to adults into their early twenties congregate in open public places (such as parks, parking lots, bridges...) to drink and socialize, as drinking in the streets is legal there. If you look in the encyclopedia (well, Wikipedia anyways...) it says that it's all about cheap inebriation in anticipation to going clubbing, where drinks are presumably more costly, but my impression (as an American exchange student in Andalucía) was that botellones existed in their own right, more for the sake of socialization than as a resource for money pressed discotheque frequenters. They vary greatly in size, the smaller more common ones taking place at night with between 4 and 15 people to a group, although if you are drinking in a popular area (usually a bar-street or convenient parking lot) your group often blends with others and passersby may observe up to a hundred kids congregated around benches down one street, surrounded by plastic bags and spilled ice. However there are also larger ones, called "macro botellones," that occur regionally (such as the Spring Festival) which start in the day time and go well into the night, and in which thousands of people participate in... certain plazas, bridges, bus stations, or other public places are swamped until about 5 AM.
The alcohol consumed is bought in the local grocery stores, usually whiskey and coke or tinto (cheap red wine also mixed with coke or any other soft drink). Technically it is illegal to sell alcohol to kids under 18, but Spaniards (at least Andalucíans)seemed pretty lax about that (the same applies to smoking). As a minor, I was only asked once to show my ID to prove my age in a grocery store and was never refused service in a bar.
However, there are some recent laws that are just coming into effect, about legal drugs in general... one of them is that after a certain time of night (it differs regionally) alcohol is no longer sold, although this law's effect is greatly diminished by all the "black market" vendors who pop up after 2AM ready to sell to the night owls.
Botellones are of course a controversy... the noise, mess, health hazard (bodily functions not taken care of in the bathroom...), and dangers of alcohol related accidents are all objected to by much of the adult population... but surprisingly, most of even the more traditional parents of the friends I made in Sevilla seemed to support the continuation of the legality of botellones, saying there was no other way provided for kids to socialize and enjoy themselves. I doubt that particular view would be shared with the parents of most of my underage American friends, and would particularly not cut it with my parents... I also noticed in a few adults who mentioned their younger, botellón attending days a kind of nostalgia, although Wikipedia also says that botellones were first noted in the 90s.
I have heard of macro botellones getting rowdy and resulting in vandalism and fights with the police, but it obviously can't be that common or they would have been illegalized long before now, and the ones I attended were peaceful (if not blurry) events.
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