BBC
An earthquake and tsunami in March knocked out vital cooling systems, triggering radiation leaks and forcing the evacuation of thousands of people.
Mr Noda's declaration of a "cold shutdown" condition marked the stabilisation of the plant.
The government says it will take decades to dismantle it completely.
The six-reactor Fukushima Daiichi nuclear plant was badly damaged by the 11 March earthquake and tsunami. Blasts occurred at four of the reactors after the cooling systems went offline.
Workers at the plant, which is operated by Tokyo Electric Power Company (Tepco), have been using sea water to cool the reactors. Waste water has built up and some contaminated liquid has been released into the sea.
A 20km (12m) exclusion zone remains in place around the plant.
'Battle not over'
"The nuclear reactors have reached a state of cold shutdown and therefore we can now confirm that we have come to the end of the accident phase of the actual reactors," Mr Noda told a news conference.
"The Japanese government promises to clarify the roadmap from here and do our utmost, while ensuring we operate the nuclear reactors as safely as possible, to decommission them."
The "battle is not over", he said, adding that the next phase would focus on the clean-up operation, including decontaminating the ground around the plant.
With the reactors stable, Mr Noda said the government would review the evacuation zones established in the immediate aftermath of the incident.
Earlier this year, the government said it was aiming for a cold shutdown by the end of the year.
This is where water that cools nuclear fuel rods remains below boiling point, meaning that the fuel cannot reheat.
Tepco has also defined it as bringing the release of radioactive materials under control and reducing public radiation exposure to a level that does not exceed 1mSv/year at the site boundary.
Speaking to cabinet ministers of his nuclear task force earlier on Friday, Mr Noda said: "We can now maintain radiation exposure at the periphery of the plant at sufficiently low levels even in the event of another accident."
But some nuclear experts have said that the repairs made to the plant after the accident are makeshift and could break down without warning.
Forty years
More than 80,000 people had to leave the area, but radiation levels in some places remain too high for them to return home.
Earlier this week, the government said it could take up to 40 years to fully decommission the plant and clean up surrounding areas.
Spent fuel rods and melted fuel inside the reactors must be removed. Waste water must also be safely stored.
Contamination has been found in foodstuffs from the region including rice, beef and fish, while radioactive soil has also been found in some areas.
Some experts have also warned that the plant could be further damaged if a powerful aftershock were to strike.
Engineers are also continuing to encounter new problems - last week Tepco officials confirmed that 45 cu m (1,590 cu ft) of water had leaked into the sea from a crack in the foundation of a water treatment facility.
Prime Minister Yoshihiko Noda: "This is a challenge to not only our nation but also the whole of humanity"
The crippled nuclear reactors at Japan's Fukushima power plant have finally been stabilised, Prime Minister Yoshihiko Noda has announced.
Mr Noda's declaration of a "cold shutdown" condition marked the stabilisation of the plant.
The government says it will take decades to dismantle it completely.
The six-reactor Fukushima Daiichi nuclear plant was badly damaged by the 11 March earthquake and tsunami. Blasts occurred at four of the reactors after the cooling systems went offline.
Workers at the plant, which is operated by Tokyo Electric Power Company (Tepco), have been using sea water to cool the reactors. Waste water has built up and some contaminated liquid has been released into the sea.
A 20km (12m) exclusion zone remains in place around the plant.
'Battle not over'
"The nuclear reactors have reached a state of cold shutdown and therefore we can now confirm that we have come to the end of the accident phase of the actual reactors," Mr Noda told a news conference.
Continue reading the main story
Retired teachers Yuji and Tetsuko Takahashi are among the tens of thousands of people forced to abandon their homes around the Fukushima plant. They were planning to watch the prime minister's announcement on the television. The set was donated to them by well-wishers like everything else in their government-provided Tokyo apartment - even the clothes on their backs.
"Even when we are told it has been shut down I can't believe it," says Tetsuko. "From the start the government officials have lied to us. One of the top officials said on TV that there was no meltdown, but it happened. The actual situation is much, much worse than we were told from the beginning."
The couple's main goal now is to return home, to get out of the 26th floor apartment and back to the garden they love. The exclusion zone could remain in force for years, but they are willing to brave the contamination. "We are old, 67 and 61," says Tetsuko . "So maybe the radiation would make the risk of getting cancer higher, but it would take five or 10 years. We are going to die before we get seriously ill."
Reaction
"Even when we are told it has been shut down I can't believe it," says Tetsuko. "From the start the government officials have lied to us. One of the top officials said on TV that there was no meltdown, but it happened. The actual situation is much, much worse than we were told from the beginning."
The couple's main goal now is to return home, to get out of the 26th floor apartment and back to the garden they love. The exclusion zone could remain in force for years, but they are willing to brave the contamination. "We are old, 67 and 61," says Tetsuko . "So maybe the radiation would make the risk of getting cancer higher, but it would take five or 10 years. We are going to die before we get seriously ill."
"We are now moving from trying to stabilise the nuclear reactors to decommissioning them.
The "battle is not over", he said, adding that the next phase would focus on the clean-up operation, including decontaminating the ground around the plant.
With the reactors stable, Mr Noda said the government would review the evacuation zones established in the immediate aftermath of the incident.
Earlier this year, the government said it was aiming for a cold shutdown by the end of the year.
This is where water that cools nuclear fuel rods remains below boiling point, meaning that the fuel cannot reheat.
Tepco has also defined it as bringing the release of radioactive materials under control and reducing public radiation exposure to a level that does not exceed 1mSv/year at the site boundary.
Speaking to cabinet ministers of his nuclear task force earlier on Friday, Mr Noda said: "We can now maintain radiation exposure at the periphery of the plant at sufficiently low levels even in the event of another accident."
But some nuclear experts have said that the repairs made to the plant after the accident are makeshift and could break down without warning.
Forty years
More than 80,000 people had to leave the area, but radiation levels in some places remain too high for them to return home.
Earlier this week, the government said it could take up to 40 years to fully decommission the plant and clean up surrounding areas.
Spent fuel rods and melted fuel inside the reactors must be removed. Waste water must also be safely stored.
Contamination has been found in foodstuffs from the region including rice, beef and fish, while radioactive soil has also been found in some areas.
Some experts have also warned that the plant could be further damaged if a powerful aftershock were to strike.
Engineers are also continuing to encounter new problems - last week Tepco officials confirmed that 45 cu m (1,590 cu ft) of water had leaked into the sea from a crack in the foundation of a water treatment facility.
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