Hackers in Brazil linked to the Anonymous collective have crashed Web sites belonging to a raft of country's leading banks, including Banco de Brasil and Citigroup.
Last week Anonymous Brazil claimed on Twitter that it was targeting the banks with denial of service attacks as part of a crusade against corruption.
The group successfully disrupted the online operations of Banco BMG, Banco Bradesco, Banco de Brasil, Banco Panamericano, HSBC, Itau Unibanco Banco Multiplo and Brazil's banking federation.
Citi spokesman Andrew Brent told the New York Times that the bank's sites in both Brazil and the US were affected, with the consumer operations down for around an hour.
Brazil's banking federation responded to the wave of hits by calling for a new law to criminalise such attacks.
Anonymous has previously targeted the sites of Bank of America, PayPal, Visa, MasterCard and others in support of WikiLeaks. In October CabinCr3w, a loose hacktivist collective associated with Anonymous, posted the personal data of Vikram Pandit, CEO of Citigroup, on the Internet.