Annan presented an apocalyptic warning as he lobbied for new, common steps after the deep divisions that opened up within the United Nations over the Iraq war in 2003.
"We must strengthen our collective defences," he told an international conference of top security officials.
"If New York or London or Paris or Berlin were hit by a nuclear terrorist attack, it might not only kill hundreds of thousands in an instant," he said, "it could also devastate the global economy, thereby plunging millions into poverty in developing nations."
The UN plans call for tougher rules to prevent the proliferation of nuclear weapons, a trust fund to help poorer countries fight terrorism, a drive to strengthen public health defences against germ warfare and a quicker action against potential threats.
Annan suggested tougher inspection rules for nuclear installations and incentives for countries to stop uranium enrichment that could be used to make nuclear bombs. He also said UN countries should adopt a common definition of terrorism and then draft an anti-terrorism convention, which should include financial help for nations to meet counterterrorism commitments.
"The United Nations must show zero tolerance of terrorism of any kind, for any reason," Annan said.
He has invited world leaders to a summit at UN headquarters in September to approve the plans.
In a more immediate appeal, he urged NATO and the European Union to do more to help end the conflict and resulting humanitarian disaster in Sudan's Darfur region.
"People are dying every single day, while we fail to protect them," he said.
German Foreign Minister Fischer broadly backed Annan's security reform plans and urged the United States - as the world's most powerful nation - to play a leading role.
Fischer also called on Washington to play a more active role in European-led diplomatic efforts to ensure Iran does not develop nuclear weapons. "If the United States were to engage positively, and I'm aware of how difficult that is, it would substantially strengthen the European drive," he said.
More broadly, he urged the European Union and the United States to work more closely as "the backbone of a new world order in the 21st century."
Annan's call for greater collective security came after U.S. Defence Secretary Donald Rumsfeld acknowledged Saturday that even the United States cannot battle terrorism and other world threats on its own.
"One nation cannot defeat the extremists alone," Rumsfeld said. "It will take the co-operation of many nations to stop the proliferation of dangerous weapons."