A student attacked the PayPal website as part of a concerted effort by the Anonymous "hacktivists" that cost the company £3.5m, a court has heard.
Christopher Weatherhead, 22, was studying at Northampton University when he allegedly took part in the campaign.
The court heard Anonymous targeted companies who opposed internet piracy but later attacked PayPal after it refused to process Wikileaks payments.
Mr Weatherhead, from Northampton, denies a charge of conspiracy.
He has pleaded not guilty to conspiring to impair the operation of computers between 1 August 2010 and 22 January 2011.
MasterCard, Visa, Ministry of Sound, the British Recorded Music Industry and the International Federation of the Phonographic Industry were also hit.
The jury at Southwark Crown Court were told Ashley Rhodes, 27, from Camberwell, south London; Peter Gibson, 24, from Hartlepool; and an 18-year-old male who cannot be named for legal reasons have already pleaded guilty to the charge.
Mr Patel said Gibson had initially suggested attacking musician Lily Allen's website because of her overt anti-piracy stance but changed his mind, saying he did not wish to "attack artists".
Sandip Patel, prosecuting, said the group caused PayPal "enormous economic harm".
He said they initially targeted companies who were known to oppose internet piracy but later switched to attacking PayPal after it refused to process payments on behalf of the controversial Wikileaks website, founded by Julian Assange.
Mr Patel said PayPal was chosen after it refused, in December 2010, to process payments for the Wau Holland Foundation, which was raising money to keep Wikileaks going.
The prosecutor said Anonymous were "hacktivists" who believed copyright should not apply to the internet.
He said their attacks, codenamed Operation Payback, began as a campaign against the music industry and those who took part in action against the Pirate Bay website which had attempted to distribute music in breach of copyright laws.
Mr Patel said they used distributed denial of service, or DDoS, which flooded the targets computers with enormous amounts of online requests.
Target websites would crash and users would be directed to a page displaying the message: "You've tried to bite the Anonymous hand. You angered the hive and now you are being stung."
Mr Patel said: "This case, simply put, is about hackers who used the internet to attack and disable computer systems - colloquially described as cyber-attackers or vandals."
He said Mr Weatherhead, who used the online name Nerdo, posted plans on an Internet Relay Chat (IRC) channel encouraging an attack on PayPal.
"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 unprecedented harm," Mr Patel added.
He said PayPal was the victim of a series of attacks "which caused considerable damage to its reputation and loss of trade".
More than 100 workers from PayPal's parent company, eBay, spent three weeks working on issues related to the attacks, said Mr Patel.
He said PayPal also had to pay for more software and hardware to defend against similar attacks in the future and he said the total cost to the firm was estimated at £3.5m.
'Dark side' of internet
Mr Patel said the case showed the "dark side" of the internet and he said Anonymous "split into organised and co-ordinated attacks almost along military lines".
The BPI was the subject of an attack in September 2010, leading it to pay out £3,996 for improved online security while the Ministry of Sound paid out £9,000 after its four websites were targeted in October 2010.
The court was told the IRC server used by Anonymous was called AnonOps, and he described Mr Weatherhead as being part of a "small cabal of leaders" and the network administrator for the group.
Mr Patel said one of the websites created by the group was also set up, paid for and run by the defendant using the fake name Moses Gustavsson.
He said Mr Weatherhead used an internet service provider (ISP) called Heihachi, which is based in Russia, which Mr Patel described as a "safe haven" for renegade ISPs.
Mr Weatherhead allegedly boasted that Heihachi permitted anything, even child pornography.
The jury was told Mr Weatherhead discussing attacking the Bank of America and the law firm GM Legal, which was involved in anti-piracy work.
The court heard that Mr Weatherhead's home was raided on 27 January 2011 and computer equipment were seized.
Mr Patel said Mr Weatherhead's security passwords were variations on the words "Nerdo is the best (or worst) hacker in the world".
He claimed the computer belonged to his sister, Laura.
The trial continues.