HSBC services around the world have suffered outages following a denial of service attack on its networks.
The global bank admitted it had been hit by a cyber attack on Friday, but insisted no customer data had been compromised during the assault.
"On 18 October 2012, HSBC servers came under a denial of service attack which affected a number of HSBC websites around the world," it said in a statement.
"This denial of service attack did not affect any customer data, but did prevent customers using HSBC online services, including internet banking."
A statement appeared on Pastebin following the attack, crediting it to the Anonymous hacktivist collective.
"As some of you may be aware HSBC bank suffered several DDoS attacks on the named sites in the past hours, us.hsbc.com hsbc.co.uk hsbc.com hsbc.ca. They were all brought down by #FawkesSecurity," it said.
It also published a video claiming credit for the attack, embedded below.
Taking down website is a common tactic used by hacktivist groups to "punish" companies. The motivation for the punishment remains unclear.
HSBC said it is taking measures to get its servers back online and had reported the incident to the authorities.
"We are taking appropriate action, working hard to restore service. We are pleased to say that some sites are now back up and running," it said.
"We are co-operating with the relevant authorities and will co-operate with other organisations that have been similarly affected by such criminal acts."
The attack is one of many to target the financial industry.
Prior to it the US Financial Services Information Sharing and Analysis Center (FS-ISAC) upgraded its Cyber Threat Advisory status from Elevated to High in September, following unexplained outages on the Bank of America and JPMorgan Chase's websites.
Despite the upgrade, US bank Wells Fargo was hit by an attack just a few days later.
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