Northampton student Christopher Weatherhead, 22, is accused of a series of cyber attacks as part of the hacking group Anonymous.
Opening the case, prosecutor Sandip Patel said:
It is the prosecution case that Christopher Weatherhead, the defendant, is a cyber attacker and that he, and others like him, waged a sophisticated and orchestrated campaign of online attacks that paralysed a series of targeted computer systems belonging to companies to which they took issue with, for whatever reason, and those attacks caused unprecendented harm.
Mr Patel said the case showed the "dark side" of the internet and said the group's attacks were "split into organised and co-ordinated attacks almost along military lines".
Weatherhead, of Holly Road, Northampton, denies a charge of conspiracy to impair the operation of computers between August 1 2010 and January 22 last year.
More top news
It will be a cooler and cloudy day with the outside chance of a shower. Highs of 16 deg C.
Becoming drier by this evening with the chance of a late sunny spell.
The Chief Executive of Southend United has told ITV News Anglia that his players deserved to win promotion to League One at Wembley.