Experts say the system, the first of its kind in the world, could save lives as it gives people time to hide under a table or in a doorway before an earthquake hits.

It could also prompt trains, elevators and factories to stop operating, limiting damage in Japan, where 20 percent of the world's big quakes are recorded and where Tokyo is said to be long overdue for a major tremor. However, private radio stations have elected not to join the scheme for six months, amid concerns it might spark panic in trains, theatres, stores and other places where crowds gather.

"If an alert was issued to subway passengers during rush hours for example, it could create huge chaos," said Takehiko Yamamura, the president of the Disaster Prevention System Institute, a private think tank.

"If someone is driving a car at 100 kph (65 mph) on a highway and receives an alert through radio and others on the same highway don't get the information, what will happen?" he said.

But the Japanese government, which has been testing the system for more than a year, says the risk of deaths and damage from quakes means it is worth going ahead. Japan still recalls with horror a quake in 1923 that killed more than 140,000 people in Tokyo, many in blazes sparked by cooking fires, while a 7.2 magnitude quake in the western port city of Kobe killed more than 6,400 people in 1995.

The Tokyo city government has said an earthquake in the capital of a similar magnitude to the Kobe one could kill more than 7,000 people and injure close to 160,000.

"The warning system is not omnipotent. This is not a system to predict or prevent earthquakes," said Toshihiro Shimoyama, a seismologist at the meteorological agency. "It can alert people only several seconds to dozens of seconds before tremors actually reach the areas they live in."

The Japan Meteorological Agency warnings will be triggered by the seismometers, placed every 25 km (16 miles) around Japan, detecting "primary" seismic waves from powerful quakes. These waves travel faster than more powerful "secondary" waves that cause most damage.

However, for those near the centre of a quake -- usually the area where damage is greatest -- there will not be enough time to trigger the warning, the agency said. As well, construction work, large explosions and lightning could prompt an alert.

As well as broadcasts, early-warning devises are being sold for 40,000 yen (US$350) upwards that count down the seconds till a quake hits, and mobile phone makers plan to sell models that will issue the warnings.

"If the operations of factories, particularly chip plants, that are vulnerable to tremors, are halted before an earthquake hits, possible damage could be vastly reduced," said Shigeki Horiuchi, a scientist at the National Research Institute for Earth Science and Disaster Prevention, who helped develop the system.

Education was essential so people knew what to expect, he added, but he said it was worth doing.

"But the system is very useful in response to offshore earthquakes that occur frequently in Japan because of distance from epicentres to inland where people live," Horiuchi said.