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Invoking memories of the bombs dropped on Japan decades ago, thousands of anti-nuclear weapon and anti-war activists marched past the United Nations, where a conference takes up the nuclear issue this week.

Chanting "No War, No Nukes" and carrying signs saying "No More Hiroshima, No more Nagasaki," the marchers ended up in Central Park (search), where they formed a human peace symbol. Organizers put the number of protesters at 40,000.

"Shut down everything — testing, nuclear weapons, the war," said Vickie Downie, of Teseque, N.M., who came with a group calling for the closing of the Los Alamos National Laboratory (search), where the bombs that devastated Hiroshima and Nagasaki were designed.

The mayor of Hiroshima, marching with a Mayors for Peace (search) group, said the survivors of the August 1945 bombs didn't want anyone else to endure what they did.

The survivors are "the only people who have had the experience of nuclear war," said Tadatoshi Akiba. "For them, the world is not one of 'us versus the enemy,' for them the world is a family and we need to work together so that no member of this family will have to suffer the pain that they suffered in 1945."

Hiroshima survivor Sunao Tsuboi was a 20-year-old college student when the bomb dropped five decades ago. Speaking through an interpreter, he told the crowd in Central Park about the physical and mental anguish he experienced.

"That's why we call the atomic bomb the absolute evil," he said.

Sessions begin Monday at the United Nations for a monthlong review of the Nuclear Nonproliferation Treaty (search), which is done every five years. The treaty calls for nation states without nuclear weapons to pledge not to pursue them, and those with such weapons to pledge to move toward eliminating them.

But there are concerns about the treaty's effectiveness, and a number of points of contention have sprung up before the review session, from North Korea's withdrawal from the treaty to the threat of nuclear terrorism.

Longtime disarmament advocate Dr. Helen Caldicott (search) said Russia and the United States are the real "rogue nations" and have enough weapons to destroy humankind.

"Let's for God's sake talk about the real moral issues of our time," she said. "Not stem cell research, gay marriage or abortion. Let's talk about whether or not the whole of the world survives, life on the planet survives."

Many protesters vented their anger over the war in Iraq. Bob Schumacher, a Vietnam War veteran from Montrose, Pa., said the public was being fed lies, as it was in the Vietnam era.

"I made a promise to people I knew who died there that I wouldn't stand by and let another war happen and not speak out," he said.

For others it was a chance for new experiences and to raise their voices in a global coalition. Seventeen-year-old Wesley Davis, of Ava, Mo., was making his first trip to New York, accompanied by his mother as he spoke out against nuclear weapons.

"Before this, I'd never even seen the ocean," he said. "Walking with people from all different countries, it's overwhelming."