There were audible sighs of relief from corporations and governments across the globe when 2000 dawned with little sign of the millennium bug and the computer meltdown doomsayers had predicted.

But barely had exhausted IT operatives put their feet up after a punishing schedule of Y2K-compliance work than a series of alarming breaches of computer security began in Japan and the United States.

It has already been a vintage year for hacking. On January 21 the celebrated hacker Kevin Mitnick was released from prison after serving five years for five fraud offences related to breaking into the computer systems of multinational giants including Sun Microsystems and Motorola.

"I saw myself as an electronic joyrider," Mr Mitnick, 36, told CBS television following his release. "I don't consider myself a thief."

Neither did the hacking community. Hackers campaigned for his release by breaking into websites and daubing them with demands to "free Kevin". A campaign website pointed out that while hackers face harsh penalties, manslaughter in the US can be punished with as little as two years in prison.

Mr Mitnick could not have taken part in the current spate of hacking even had he desired to - his release conditions forbid him access to any computer. But there is a growing community of young hackers carrying on where he left off. Ira Winkler, the president of the Information Security Advisory Group, estimates that the number of hackers worldwide has grown from 35,000 two years ago to 100,000 today.

Late in January hackers began a high-profile campaign to crack open Japanese government websites. On at least five sites, links were inserted to porn websites.

The Japanese government's computer systems are widely viewed as vulnerable compared to those in the US. Tokyo has pledged to tighten security to US levels over the next three years and announced the creation of a 30-strong special police force to combat hackers. A digital signature bill aimed at verifying internet users' identities is also to be submitted to parliament.

This spate of hacking has coincided with a new law which came into force in Japan this month making it illegal to access sites without the proper clearance. As the US has found, all attempts to tighten security on the internet serve to redouble the efforts of those who seek to break and enter computer systems or demonstrate free speech on the internet.

But the hacking that brought down a host of bluechip US websites, including Yahoo! and Amazon, seems not to be part of any political manifesto but is more the result of a new phenomenon: the rise of the "script kiddie".

Dozens of US teenagers have been accused of internet vandalism in the past few months alone. A group of teenagers recently boasted to the US media that they steal AOL identities and masquerade as other users "just for the pure joy of trying to ruin friendships by insulting friends who have no idea they are talking to a hacker and not the victim". They also caused a serious credit card scare by obtaining users' card details.

And in Norway in January a 16-year-old was arrested for allegedly creating and distributing a software programme which enables the piracy of DVD movies.

Another teenager, calling himself "Maxus", hacked into CD Universe's computers, stealing 300,000 credit card numbers. Maxus began publishing the company's customers' card numbers on his website after it refused his demand for a "Christmas present" of £62,000.

Most of these teenage cyber criminals broke or entered into corporate computers by parroting relatively simple computer "scripts" picked up freely on the internet. The "denial of service" attacks brought down Yahoo! and others by swamping the corporations' servers with multiple requests for information, so that other users could not access the site. Denial of service can also be created by "mail bombing" - flooding computer servers with mass emails.

"What we're seeing is adolescent pranks going mainstream," observed the MIT network manager, Jeff Schiller. The US justice department is attempting to stop children growing up to be script kiddies with a $300,000 (£186,000) cybercitizen partnership campaign, aimed at promoting responsible, law-abiding surfing.

Many script kiddies in the US are caught swiftly, often because they boast of their hacking exploits on the internet or are betrayed by their internet names.

Jay Satiro, 19, was jailed for a year after hacking into AOL's computers and replacing programmes with his own. He was banned from using a PC for five years.

But hackers are able to cross national boundaries with ease. After the FBI was called in to investigate Maxus's attempts at extortion, a CD Universe spokesman, Brett Brewer, said the hacker had been identified as a teenager in Russia, but had not been apprehended.

The elitist hacking community distances itself from the script kiddies. A statement by Hacker Quarterly described the recent attacks on corporate US sites as "malicious assaults on infrastructure" which "have nothing to do with hacking at all".

Hackers prefer the term "crackers" to describe those who break and enter into computers illegally to vandalise internet property or steal information.

"The world of hackers is 95% non-criminal," the US-based hackers' thinktank L0pht claims. "It is a world of people exploring the edges of technology and building things. The crazy thing is, the government is making more and more of that exploration illegal."

The "hacker ethic" heralds "information sharing" as a "powerful positive good".

This contrasts, hackers say, with crackers, who tend to gather in tight-knit, secretive groups. Crackers are proud of their sophisticated code-cracking - but they keep quiet about it, unlike the hacking community, who make no secret of their distaste for government regulation of the internet and the lucrative hardware and software businesses run by Microsoft, IBM and others.

The current spate of hacking will spur corporate America to close up computer "back doors" - the crackers' holy grail that allows them access to a computer system as often they like, even when the security system is enhanced or updated.

But as long as America's computer-literate teens can cripple multinational and government websites, the glamour of hacking will continue to lure script kiddies to have a crack at infamy.