By Taiga Uranaka and Ki Joon Kwon
FUKUSHIMA, Japan (Reuters) - Japan warned radioactive levels had become "significantly" higher around a quake-stricken nuclear power plant on Tuesday after explosions at two reactors, and the French embassy said a low-level radioactive wind could reach Tokyo within hours.
Prime Minister Naoto Kan urged people within 30 km (18 miles) of the facility north of Tokyo to remain indoors, underscoring the dramatic escalation of Japan's nuclear crisis, the world's most serious since the Chernobyl disaster in Ukraine in 1986.
As concern about the crippling economic impact of the nuclear and earthquake disasters mounted, Japanese stocks plunged more than 14 percent - heading for their biggest drop since 1987 -- compounding a slide of 7.6 percent the day before. The two-day fall has wiped $720 billion off the market.
In a sign of mounting fears about the risk of radiation, neighboring China said it was strengthening monitoring and Air China said it had canceled flights to Tokyo.
Several embassies advised staff and citizens to leave affected areas. Tourists cut short vacations and multinational companies either urged staff to leave or said they were considering plans to move outside the city.
But many Japanese were cancelling trips and staying put, saying now is not the time to leave home.
"There has been a fire at the No. 4 reactor and radiation levels in the surrounding area have heightened significantly. The possibility of further radioactive leakage is heightening," a grim-faced Kan said in an address to the nation.
"We are making every effort to prevent the leak from spreading. I know that people are very worried but I would like to ask you to act calmly."
As concerns rose over the nuclear accident, Japanese media became more critical of Kan's handling of the disaster and criticized the government and plant operator TEPCO for their failure to provide enough information on the incident.
The French Embassy in Tokyo warned in an 0100 GMT advisory that a low level of radioactive wind could reach the capital -- 240 km (150 miles) south of the plant -- in about 10 hours.
Winds over the facility were blowing slowly in a southwesterly direction that includes Tokyo but will shift westerly later on Tuesday, a weather official said.
Kyodo news agency said radiation levels nine times normal levels had been briefly detected in Kanagawa near Tokyo, but it quotes the metropolitan government that only "minute levels" were found in the capital itself.
"Very acute radiation, like that which happened in Chernobyl and to the Japanese workers at the nuclear power station, is unlikely for the population," said Lam Ching-wan, a chemical pathologist at the University of Hong Kong.
But the blasts could expose the population to longer-term exposure to radiation, which can raise the risk of thyroid and bone cancers and leukemia. Children in fetuses are especially vulnerable.
There have been a total of four explosions at the plant since it was damaged in last Friday's massive quake and tsunami. The most recent were blasts at reactors No. 2 and No. 4.
Authorities had previously been trying to prevent meltdowns in the Fukishima Daiichi complex's nuclear reactors by flooding the chambers with sea water to cool the reactors down.
There has been panic buying in Tokyo. Don Quixote, a multi-storey, 24-hour general store in Roppongi district, sold out of radios immediately after the quake. It also sold out of flashlights, candles, petrol containers and sleeping bags.
The full extent of the destruction from last Friday's massive quake and tsunami that followed it was still becoming clear, as rescuers combed through the region north of Tokyo where officials say at least 10,000 people were killed.
"It's a scene from hell, absolutely nightmarish," said Patrick Fuller of the International Red Cross Federation from the northeastern coastal town of Otsuchi.
Kan has said Japan is facing its worst crisis since World War Two and, with the financial costs estimated at up to $180 billion, analysts said it could tip the world's third-biggest economy back into recession.
The U.S. Geological Survey upgraded the quake to magnitude 9.0, from 8.9, making it the world's fourth most powerful since 1900.
Car makers, shipbuilders and technology companies worldwide scrambled for supplies after the disaster shut factories in Japan and disrupted the global manufacturing chain.
BLAST DAMAGES ROOF, WORKERS TOLD TO LEAVE
The fear at the Fukushima plant is of a major radiation leak after the quake and tsunami knocked out cooling systems.
Jiji news agency said the first explosion on Tuesday damaged the roof and steam was rising from the complex. Some workers were also told to leave the plant, a development one expert had warned beforehand could signal a worsening of the crisis.
The accident and the response have prompted criticism that authorities were ill-prepared and revived debate in many countries about the safety of atomic power.
"You're above Three Mile Island but you're nowhere near a Chernobyl ... Chernobyl there was no impediment to release, it just blew everything out into the atmosphere," said Murray Jennex, professor at San Diego State University in California.
"You've still got a big chunk of the containment there holding most of it in."
Switzerland put on hold some approvals for nuclear power plants and Germany said it was scrapping a plan to extend the life of its nuclear power stations. The White House said U.S. President Barack Obama remained committed to nuclear energy.
Whilst the Fukuskima plant's No.1 and No.3 reactors both suffered partial fuel rod meltdowns, operator Tokyo Electric Power Co (TEPCO) had earlier said the No.2 reactor was now the biggest concern.
A sudden drop in cooling water levels when a pump ran out of fuel had fully exposed the fuel rods for a time, an official said. This could lead to the rods melting down and a radioactive leak.
TEPCO had resumed pumping sea water into the reactor early on Tuesday.
U.S. warships and planes helping with relief efforts moved away from the coast temporarily because of low-level radiation. The U.S. Seventh Fleet described the move as precautionary.
South Korea, Hong Kong, Singapore and the Philippines said they would test Japanese food imports for radiation.
About 850,000 households in the north were still without electricity in near-freezing weather, Tohuku Electric Power Co. said, and the government said at least 1.5 million households lack running water. Tens of thousands of people were missing.
"The situation here is just beyond belief, almost everything has been flattened," said the Red Cross's Fuller in Otsuchi, a town all-but obliterated. "The government is saying that 9,500 people, more than half of the population, could have died and I do fear the worst."
Whole villages and towns have been wiped off the map by Friday's wall of water, triggering an international humanitarian effort of epic proportions.
Estimates of the economic impact are now starting to emerge.
Hiromichi Shirakawa, chief economist for Japan at Credit Suisse, said in a note to clients that the economic loss will likely be around 14-15 trillion yen ($171-183 billion) just to the region hit by the quake and tsunami.
Even that would put it above the commonly accepted cost of the 1995 Kobe quake which killed 6,000 people.
The earthquake has forced many firms to suspend production and global companies -- from semiconductor makers to shipbuilders -- face disruptions to operations after the quake and tsunami destroyed vital infrastructure, damaged ports and knocked out factories.
"The earthquake could have great implications on the global economic front," said Andre Bakhos, director of market analytics at Lec Securities in New York. "If you shut down Japan, there could be a global recession."
(Additional reporting by Nathan Layne, Risa Maeda and Leika Kihara in Tokyo, Chris Meyers and Kim Kyung-hoon in Sendai, Noel Randewich in San Francisco, Tan Ee-lyn in Hong Kong and Miyoung Kim in Seoul; Writing by Nick Macfie; Editing by John Chalmers)