Suspecting the base houses US nuclear weapons, the campaigners for a nuclear-free zone hauled themselves over two levels of security barriers around 0730 (0530 GMT).

The Kleine-Brogel base houses around 20 of a 183-strong remaining post Cold War stockpile of US nuclear bombs in Europe, according to estimates by Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists.

"They entered holding aloft banners," Agir pour la Paix said in a statement as spokesman Stephanie Demblon recognised the trespassers could face five years in jail for "civil disobedience".

The NGO wants the Belgian government to establish "a roadmap for the withdrawal of nuclear weapons from Belgian territory" and to sign the UN Non-Proliferation Treaty.

Belgian television showed the protesters using a ladder to clamber over wire-topped fences and reported all eight were released several hours later but now face prosecution.