"Tibet is an inseparable part of China," Chinese foreign ministry spokeswoman Hua Chunying said. "We are firmly opposed to any country or any individuals' interference in China's domestic affairs under the pretext of the Tibet-related issue."
The Spanish court's decision on Thursday followed an appeal by Tibetan exile groups against a June ruling by one of its judges to drop the case.
Spanish judicial system recognises universal justice principle under which a genocide suspect can be put on trial outside their country. But mandates that such a case must directly involve a Spanish national.
Hua also stressed that China and Spain have "friendly relations" for which both sides "attach great importance".
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She said that the organisation behind the lawsuit is "attempting to attack the Chinese government and sabotage friendly relations" between China and Spain.
The Spanish legal system recognises the universal justice principle, under which genocide suspects can be put on trial outside their home country.
But mandates that at least one victim of alleged genocide must be a Spanish citizen.