SPAIN is going on a bender tonight. But if you are not a teenager, you aren’t invited.

More than half a million youths are expected to take part in the country’s biggest botellón, a national drinking binge, as cities from Málaga to Madrid and Barcelona to Bilbao try to outdo one another by staging the largest gathering.

Word has spread by e-mail and text messages, and under-age drinkers are expected to crowd into city centres armed with bottles of Coca-Cola or other mixers generously spiked with alcohol for this mass outdoor “macro-botellón”.

The event has caused alarm among city authorities, who are struggling to make arrangements to cater for the vast numbers expected to take part. Police leave has been cancelled amid fears that drunken youths will turn to violence.

The Government came under pressure to introduce emergency legislation to curb under-age drinking. But this is unlikely to have much impact, not least because across Spain regional authorities have set different minimum ages to buy alcohol. Regional bylaws combating under-age drinking also vary widely, making it difficult for Madrid to stop what is perceived as a national problem.

Elena Salgado, the Health Minister, yesterday called on parents to keep their children under control. “We have already seen the fall in drink-driving, but the danger of under-age drinkers is much worse,” she said.

Despite Spain’s reputation for civilised Mediterranean drinking habits, many teenagers totter home after a botellón, leaving city centres strewn with rubbish, and residents, who have been kept up all night, furious.

The botellón, or “big bottle”, began in Madrid in the 1980s. But in recent months the new and more worrying phenomenon of the macro-botellón has started to worry Spanish parents and authorities. Children as young as 12 gather on the streets to be at the biggest botellón.

Lorenzo Navarette, a sociologist and author of The Phenomenon of the Botellón, said: “The macro-botellón is nothing more than the result of a situation which has been out of control. Now it is overwhelming.” Señor Navarette added that Spanish teenagers would want to provoke the authorities into reacting to tonight’s mass-drinking session. “The problem is they only socialise through alcohol.” City councils refused to disclose the number of police who would be deployed on the streets “for fear of provoking a confrontation”.

Under-age drinkers claim that this way they can enjoy a night out for as little as €6 (£4) a night, whereas bars are too expensive.

They complain that there is nowhere for them to go. They do not share the British habit of drinking until they drop, but many could end the night in hospital, doctors said.

Dr Darío Fernández said: “I hope councils are prepared for this, because a percentage of those who go to the botellón will finish up in intensive care.”

Spanish teenagers have been steadily drinking more. A national drugs survey conducted in 2004 indicated that 38 per cent of 14-year-olds drank alcohol regularly.

Estanía Marín, 19, from Seville, who has been going to el botellón since she was 14, admits that many of her friends drink too much. “Some keep it under control, but generally most don’t,” she said.

“But I do this because I can meet friends, it’s cheaper and only costs €5 between all of us for a bottle, whereas in bars it is €8 for each drink.”