The rights group World Organisation Against Torture (OMCT) has called on Swiss authorities to open an investigation into former US President George W. Bush if he sets foot on Swiss soil next week.

Bush has reportedly been invited to a fund-raising gala evening by a Jewish organisation on February 12 in the Swiss city of Geneva, prompting an outcry by several groups about the use of "torture" during interrogations of US "war on terror" detainees.

The Geneva-based OMCT on Thursday released a letter it sent to Swiss President and Foreign Minister Micheline Calmy-Rey to underline Switzerland's obligation under domestic law and the UN Convention Against Torture to investigate such allegations.

"This would have to include also former President G (W) Bush who had the overall control as commander in chief and as all information suggests authorised, knew and acquiesced into the practices that constitute the crime of torture," the letter said.

It outlined an "overhwelming" body of evidence relating to the use of a "wide array of physical and psychological abuse" including "water boarding" and sleep deprivation, as well as "recent admissions" in Bush's memoirs.

The former US commander-in-chief claimed in his memoirs published last year that use of the water boarding technique -- which simulates drowning -- had directly prevented terrorist attacks in Britain and the United States.

Bush also said in an interview with the British newspaper The Times it was "damn right" that he had authorised use of the controversial method on Al-Qaeda's 9/11 mastermind Khalid Sheikh Mohammed.

"There are no laws that provide an exception for former head of states," OMCT Secretary General Eric Sottas said in a statement.

"There is not the slightest doubt that Switzerland is obliged under both international law and its domestic law to initiate an investigation for acts of torture against any individual present on its territory that has committed, authorised, participated in or was otherwise complicit in acts of torture," he added.

The United Nations convention covers "torture and other cruel, inhuman or degrading treatment or punishment". Both Switzerland and the United States have ratified the pact.

Several mainly left wing groups have called for demonstrations on February 12.