According to Japanese officials, the explosion injured at least four workers at the Fukushima Dai-ichi nuclear plant, and the 40-year-old facility was already facing a possible meltdown. The walls of one building were also destroyed.
"We are now trying to analyze what is behind the explosion," said government spokesman Yukio Edano. "We ask everyone to take action to secure safety."
Large amounts of radiation were spewing out and the evacuation area around the plant was expanded but officials did not know how dangerous the leak was to people. Shinji Kinjo, a spokesman for the Japanese nuclear agency, could not say how much radiation was in the atmosphere or how hot the reactor was following the failure of its cooling system.
It is unclear if the building that was damaged by the blast contained the reactor, but Tokyo Electric said an explosion happened in the first reactor, according to local media reports.
Images on Japanese television showed the crumbled remains of one of the plant building's walls with smoke emerging from the site.
The incident came as the level of water used to cool a nuclear reactor dropped to an alarming level Saturday, heightening fears of a larger nuclear disaster.
"They're not being designed to withstand the worst case accidents. They're only being built to withstand the most likely. If you do that, eventually you're going to hit a worse case," said nuclear policy expert Joe Cirincione.
Japan's nuclear and industrial safety agency says the plant is releasing "radioactive vapors," and the pressure in the reactor is now twice the normal level. And if the fuel rods remain exposed, they will be damaged, releasing radioactivity.
The plant is 200 miles northeast of Tokyo. Saturday morning, the Japanese government had evacuated civilians from a 13-mile radius around the plant.
Some experts were saying the explosion could be the worst nuclear disaster since Chernobyl.
Friday's double disaster of an earthquake and tsumani that pulverized Japan's northeastern coast, has left 574 people dead by official count, although local media reports said at least 1,300 people may have been killed.
In Chicago, the staff at the Consulate General of Japan's office is watching for news of the earthquake. Representatives say many Japanese businesses invest in the Midwest, and they could be impacted by the disaster.
Also, Osaka, Japan is one of Chicago's sister cities and has an office in Chicago. The staff has been using social media to check on people in Japan.
Operations should be back to normal between Chicago's O'hare Airport and Tokyo Saturday. Because of the devastation in Japan, American Airlines had canceled its flights from Chicago. United's afternoon flight took off for Tokyo with just a few people on board Friday.
American says passengers ticketed on flights to Japan between Saturday and Monday can change their itineraries without penalty.
The Red Cross has set up a disaster relief fund. Those wanting to donate can text "redcross" to 90999. A $10 donation will go to support relief efforts for the earthquake in Japan and tsunami throughout the Pacific.
The trouble began at the nuclear plant's Unit 1 after the massive 8.9-magnitude earthquake and the tsunami it spawned knocked out power there. According to official figures, 586 people are missing and 1,105 injured. In addition, police said between 200 and 300 bodies were found along the coast in Sendai, the biggest city in the area near the quake's epicenter.
The true scale of the destruction was still not known more than 24 hours after the quake since washed-out roads and shut airports have hindered access to the area. An untold number of bodies were believed to be buried in the rubble and debris.
In another disturbing development that could substantially raise the death toll, Kyodo news agency said rail operators lost contact with four trains running on coastal lines on Friday and still had not found them by Saturday afternoon.
East Japan Railway Co. said it did not know how many people were aboard the trains.
Adding to worries was the fate of nuclear power plants. Japan has declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability.
The most troubled one, Fukushima Dai-ichi, is facing meltdown, officials have said.
A "meltdown" is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant's systems and its ability to manage temperatures. It is not immediately clear if a meltdown would cause serious radiation risk, and if it did how far the risk would extend.
Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely.
"It's not a fast reaction like at Chernobyl," he said. "I think that everything will be contained within the grounds, and there will be no big catastrophe."
In 1986, the Chernobyl nuclear reactor exploded and caught fire, sending a cloud of radiation over much of Europe.
Pressure has been building up in Fukushima reactor -- it's now twice the normal level -- and Japan's Nuclear and Industrial Safety Agency told reporters Saturday that the plant was venting "radioactive vapors." Officials said they were measuring radiation levels in the area. Wind in the region is weak and headed northeast, out to sea, according to the Meteorological Agency.
The reactor in trouble has already leaked some radiation: Before the explosion, operators had detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.
Also before the blast, Ryohei Shiomi, a nuclear official, said that each hour the plant was releasing the amount of radiation a person normally absorbs in a year.
"Everyone wants to get out of the town. But the roads are terrible," said Reiko Takagi, a middle-aged woman, standing outside a taxi company. "It is too dangerous to go anywhere. But we are afraid that winds may change and bring radiation toward us."
Meanwhile, the first wave of military rescuers began arriving by boats and helicopters.
Prime Minister Naoto Kan said 50,000 troops would join rescue and recovery efforts following the quake that unleashed one of the greatest disasters Japan has witnessed -- a 23-foot (7-meter) tsunami that washed far inland over fields, smashing towns, airports and highways in its way.
"Most of houses along the coastline were washed away, and fire broke out there," said Kan after inspecting the quake area in a helicopter. "I realized the extremely serious damage the tsunami caused."
More than 215,000 people were living in 1,350 temporary shelters in five prefectures, or states, the national police agency said. Since the quake, more than 1 million households have not had water, mostly concentrated in northeast.
The transport ministry said all highways from Tokyo leading to quake-hit areas were closed, except for emergency vehicles. Mobile communications were spotty and calls to the devastated areas were going unanswered .
Local TV stations broadcast footage of people lining up for water and food such as rice balls. In Fukushima, city officials were handing out bottled drinks, snacks and blankets. But there were large areas that were surrounded by water and were unreachable.
One hospital in Miyagi prefecture was seen surrounded by water. The staff had painted an SOS on its rooftop and were waving white flags.
Kan said a total of 190 military aircraft and 25 ships have been sent to the area, which continued to be jolted by tremors, even 24 hours later.
More than 125 aftershocks have occurred, many of them above magnitude 6.0, which alone would be considered strong.
Technologically advanced Japan is well prepared for quakes and its buildings can withstand strong jolts, even a temblor like Friday's, which was the strongest the country has experienced since official records started in the late 1800s. What was beyond human control was the killer tsunami that followed.
It swept inland about six miles (10 kilometers) in some areas, swallowing boats, homes, cars, trees and everything else.
"The tsunami was unbelievably fast," said Koichi Takairin, a 34-year-old truck driver who was inside his sturdy four-ton rig when the wave hit the port town of Sendai.
"Smaller cars were being swept around me," he said. All I could do was sit in my truck."
His rig ruined, he joined the steady flow of survivors who walked along the road away from the sea and back into the city on Saturday. Smoke from at least one large fire could be seen in the distance.
Smashed cars and small airplanes were jumbled up against buildings near the local airport, several miles (kilometers) from the shore. Felled trees and wooden debris lay everywhere as rescue workers coasted on boats through murky waters around flooded structures, nosing their way through a sea of debris.
Basic commodities were at a premium. Hundreds lined up outside of supermarkets, and gas stations were swamped with cars. The situation was similar in scores of other towns and cities along the 1,300-mile-long (2,100-kilometer-long) eastern coastline hit by the tsunami.
In Sendai, as in many areas of the northeast, cell phone service was down, making it difficult for people to communicate with loved ones.
President Barack Obama pledged U.S. assistance following what he called a potentially "catastrophic" disaster. He said one U.S. aircraft carrier was already in Japan and a second was on its way. A U.S. ship was also heading to the Marianas Islands to assist as needed, he said.
Japan's worst previous quake was a magnitude 8.3 temblor in Kanto that killed 143,000 people in 1923, according to the USGS. A magnitude 7.2 quake in Kobe killed 6,400 people in 1995.
Japan lies on the "Ring of Fire" -- an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world's quakes occur, including the one that triggered the Dec. 26, 2004, Indian Ocean tsunami that killed an estimated 230,000 people in 12 countries. A magnitude-8.8 quake that shook central Chile in February 2010 also generated a tsunami and killed 524 people.
(The Associated Press contributed to this report.)