Remembrance ceremonies will be held in towns and cities around the disaster zone and in the capital Tokyo, where Emperor Akihito and Empress Michiko are to lead tributes to those who lost their lives in Japan's worst peace-time disaster.
Many local governments will switch on a tsunami alarm siren at 2:46 pm (0546 GMT), marking the exact moment a 9.0-magnitude undersea quake hit.
Its raw force unleashed a towering tsunami that travelled at the speed of a jet plane to the coast. Within minutes, communities were turned to matchwood, and whole families had drowned.
The crippled plant remains volatile and experts say the complicated decommissioning process will take decades, as fears persist over the long-term health effects of leaked radiation. The accident forced tens of thousands to flee from areas around the shattered site.
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Although no one died as a direct result of Fukushima, about 1,650 area residents died from complications related to stress and other problems following the accident.
A total of 15,884 people are confirmed to have died in the tsunami with another 2,636 unaccounted for. Searchers still find human remains.
Among almost 270,000 evacuees from the tsunami and Fukushima, about 100,000 are in temporary housing while others found shelter in new cities or with relatives.