A moment of silence today marked one year since a catastrophic earthquake and tsunami battered Japan's northeast coast, and triggered the worst nuclear disaster since Chernobyl.

Sirens blared across Japan at 2:46 p.m. as the nation stood still, the moment disaster struck. The magnitude-9.0 earthquake unleashed a powerful tsunami that towered more than 120 feet in some areas. The catastrophic waves flattened entire communities in the Tohoku region and killed nearly 20,000 people, though more than 3,000 bodies are still missing.

At a memorial service in Tokyo's National Theater, Emperor Akihito and Empress Michiko bowed their heads in silence, along with hundreds of others attending the ceremony.

"I ask that you keep the victims in your thoughts," he said. "If we all work hard, I am hopeful the situation will improve in the devastated regions."

Mourners gathered early to honor victims in the hardest hit regions.

In the ravaged city of Ishinomaki, where more than 3,000 people died, residents offered prayers at a Buddhist temple, before dawn. They began ringing the bell - 19,000 times - for every life lost.

In Rikuzentakata, a fishing town that lost more than 1,500 people, survivors gathered at the "tree of hope" to reflect on the horrors from one year ago.

Others marked the anniversary alone, quietly kneeling down on barren land where their homes once stood. "I wanted to save people but I couldn't," said Naomi Fujino, who lost her father to the tsunami. "I couldn't even help my father. What can I do but keep going."

While much of the debris has been swept aside into large piles, reconstruction has yet to begin in many towns.

About 325,000 people remain homeless, living in temporary housing units while the local and central government debate where and how to rebuild. In many communities, the opinions are split. Longtime residents, many of them elderly, want to return to the same areas the tsunami wiped out. Others want to build on higher ground, even if the process takes longer.

In areas around the Fukushima plant, hope for any homecoming is fading fast. About 80,000 people living within a 12-mile radius were evacuated, when the tsunami knocked out vital cooling systems, and triggered three reactors meltdowns that spewed radiation into the air.

Today, that nuclear exclusion zone remains off limits because of high radiation levels. Residents have only been allowed back home for a handful of temporary, supervised visits, and might not be able to return permanently for decades.

"We have no prospects for the future," said Katsunobu Sakurai, mayor of Minamisoma, a city that partially sits inside the exclusion zone. "We have been handed a life filled with anxiety. No amount of money can change that.

The government has said it will take at least 30 years to decommission the nuclear plant, a tedious process that requires removing melted nuclear fuel from the core and disposing spent fuel rods.

Meanwhile, cities just outside the plant have been turned into nuclear wastelands, resembling little signs of life. Crews have begun removing top soil and pressure washing homes and buildings to reduce radiation levels as part of an ambitious decontamination project that is expected to take decades.

In communities outside the zone, cleanup has been stalled by a failure to secure a nuclear waste site.

While there have been no reports of radiation-related illnesses or deaths, the emperor said Sunday that Japan faced an uphill battle, in efforts to rid Fukushima of contamination.

"In order to make the area inhabitable again, we face the difficult problem of removing radiation," he said. "We shall not let our memory of the disasters fade, pay attention to disaster prevention and continue our effort to make this land even a safer place to live."

Speaking at the same memorial service, Prime Minister Noda renewed his commitment to rebuild the devastated region, saying Japan had repeatedly risen from crisis.

"We will stand by the people from the disaster-hit areas and join hands to achieve this historic task to rebuild," he said.