The International Consortium of Investigative Journalists (ICIJ), which coordinated the reporting, said a list of people who held HSBC accounts in Switzerland included soccer and tennis professionals, rock stars and Hollywood actors.
Reuters could not independently verify any of the names listed by the ICIJ. Having a Swiss bank account is not illegal and many are held for legitimate purposes.
The client list included royalty such as Morocco's King Mohammed, politicians, corporate executives including former Santander chairman Emilio Botin, who died last year, and wealthy families, the ICIJ said. A spokesman for the Moroccan royal palace declined to comment.
It also listed arms dealers, people linked to former dictators and traffickers in blood diamonds, and several individuals on the current U.S. sanctions list, including Gennady Timchenko, an associate of Russian President Vladimir Putin. Timchenko's Volga Group declined to comment.
"We acknowledge and are accountable for past compliance and control failures," HSBC said late on Sunday after news outlets published the allegations about its Swiss private bank.
The Guardian, along with other news outlets, cited documents obtained by the ICIJ via French newspaper Le Monde.
HSBC said that its Swiss arm had not been fully integrated into HSBC after its purchase in 1999, allowing "significantly lower" standards of compliance and due diligence to persist.
The Guardian asserted that the files showed HSBC's Swiss bank routinely allowed clients to withdraw "bricks" of cash, often in foreign currencies which were of little use in Switzerland.
HSBC also marketed schemes which were likely to enable wealthy clients to avoid European taxes and colluded with some to conceal undeclared accounts from domestic tax authorities, the Guardian added.
The reports triggered political debate in Britain ahead of a parliamentary election in May. Margaret Hodge, a senior opposition Labor Party lawmaker, said UK tax authorities had done too little.
"All the other countries have collected much more," she told BBC Radio on Monday. "We are never assertive enough, aggressive enough to protect the taxpayer."