Four workers were injured after the blast at the Fukushima Daiichi plant, Chief Cabinet Secretary Yukio Edano told reporters. It was not immediately clear where the blast occurred inside the plant, or what caused it.
The roof of a reactor at the plant collapsed following the explosion around 3:30 p.m., Japan's Kyodo News Agency reported, citing Tokyo Electric Power Company.
One expert said the explosion was "clearly a serious situation," but may not be related to problems inside the plant's nuclear reactor.
Other effects of the tsunami may have caused the blast, said Malcolm Grimston, associate fellow for energy, environment and development at London's Chatham House.
"It's clearly a serious situation, but that in itself does not necessarily mean major (nuclear) contamination," he said.
Japanese public broadcaster NHK said the injured workers were in the process of cooling a nuclear reactor at the plant by injecting water into its core.
The Fukushima prefecture government said hourly radiation levels at the plant had reached levels allowable for ordinary people over the course of a year, Kyodo reported.
Earlier Saturday, Japan's nuclear agency said workers were continuing efforts to cool fuel rods at the plant after a small amount of radioactive material escaped into the air.
The agency said there was a strong possibility that the radioactive cesium monitors detected was from the melting of a fuel rod at the plant, adding that engineers were continuing to cool the fuel rods by pumping water around them.
Cesium is a byproduct of the nuclear fission process that occurs in nuclear plants.
A spokesman for Japan's Nuclear and Industrial Agency earlier said atomic material had seeped out of one of the five nuclear reactors at the Daiichi plant, located about 160 miles (260 kilometers) north of Tokyo.
Authorities evacuated people living near the reactor after an earthquake and tsunami crippled cooling systems there, as well as at another Tokyo Electric Power Company nuclear plant in Japan's Fukushima prefecture.
"This is a situation that has the potential for a nuclear catastrophe. It's basically a race against time, because what has happened is that plant operators have not been able to cool down the core of at least two reactors," said Robert Alvarez, a senior scholar at the Institute for Policy Studies in Washington.
By late Saturday, authorities had extended the evacuation area to 20 kilometers around the Daiichi plant, Kyodo reported.
The evacuations notwithstanding, the nuclear safety agency asserted Saturday that the radiation at the plants did not pose an immediate threat to nearby residents' health, the Kyodo News Agency said.
The International Atomic Energy Agency said Friday on its website that the quake and tsunami knocked out a Daiichi reactor's off-site power source, which is used to cool down the radioactive material inside. Then, the tsunami waves disabled the backup source -- diesel generators -- and authorities were working to get these operating.
On Saturday, Japanese nuclear authorities said the cooling system had also failed at three of the four reactors at the Fukushima Daini plant -- located in another town in northeaster Japan's Fukushima Prefecture.
Authorities also ordered the release of valves at affected reactors at the two plants Saturday -- a move that experts said was likely done to release growing pressure inside as high temperatures caused water to boil and produce excess steam.
Janie Eudy told CNN that her 52-year-old husband, Joe, was working at the Daiichi plant and was injured by falling and shattering glass when the quake struck. As he and others were planning to evacuate, at their managers' orders, the tsunami waves struck and washed buildings from the nearby town past the plant.
"To me, it sounded like hell on earth," she said, adding her husband -- a native of Pineville, Louisiana -- ultimately escaped.
The power company reported Saturday that about 1 million households were without power, and that power shortages may occur due to damage at the company's facility.
"We kindly ask our customers to cooperate with us in reducing usage of power," the company said.
James Acton, a physicist who examined the Kashiwazaki-Kariwa plant after a 2007 earthquake, told CNN that releasing the valves at the two power plants might spew a relatively small amount of radioactive material into the atmosphere.
"The big problem is if it can't cool and the (reactors') core starts to melt -- then you have the possibility of a greater release of radioactivity into the environment," Acton said. If that happens, "there's a possibility of cancer in the long term -- that's the main hazard here."