But governments insisted they did not know the true nature of the threat because there was no international system in place to track tidal waves in the Indian Ocean — an area where they are rare — and they can't afford to buy sophisticated equipment to build one.
And what warnings there were came too little, too late.
Scientists say the huge death toll could have been avoided if the region had had a warning system in place.
There is an international warning system that has proved effective in warning of impending disaster. But its members are mostly in areas where tidal waves are more common, like the Pacific Rim.
Thailand is a member, but it has no sensor buoys on its west coast, where the waves struck.
A scientist with the U.S. Geological survey says the wave sensors won't show how big a tidal wave might be, but they can predict when they will hit land, allowing authorities to warn citizens to get off the coast.
The magnitude 9.0 earthquake — the largest in 40 years — shifted huge geological plates beneath the sea northwest of Sumatra island, causing a massive and sudden displacement of millions on tons of water.
Indonesia villages closest to the temblor's epicenter were swamped within minutes, but elsewhere the waves radiated outwards, gathering speed and ferocity until they made landfall.
Waves began pummeling southern Thailand about one hour after the earthquake. After 2½ hours, the torrents had traveled some 1,000 miles and slammed India and Sri Lanka. Malaysia, the Maldives, Myanmar, and Bangladesh were also hit. Eventually they struck Somalia, on the east coast of Africa.
Indonesian officials said they had no way to know that the earthquake had caused the earthquake-driven waves, or tsunamis, or how dangerous they might have been.
"Unfortunately, we have no equipment here that can warn about tsunamis," said Budi Waluyo, an official with Indonesia's Meteorology and Geophysics Agency. "The instruments are very expensive and we don't have money to buy them."
But Thammasarote Smith, a former senior forecaster at Thailand's Meteorological Department, said governments could have done much more to warn people about the danger.
"The department had up to an hour to announce the emergency message and evacuate people but they failed to do so," Thammasarote was quoted as saying in The Bangkok Post newspaper. "It is true that an earthquake is unpredictable but a tsunami, which occurs after an earthquake, is predictable."
Kathawudhi Marlairojanasiri, the department's chief weather forecaster, said it had sent out warnings through radio and television from 9 a.m. on Sunday about a possible undertow along the southwest coast of Thailand, where tens of thousands of foreign tourists were vacationing.
But the warnings came after the first waves hit. A Web site warning went up three hours later — but by then, at least 700 people had died in Thailand, including a jet-skiing grandson of revered monarch King Bhumibol Adulyadej.
Sulamee Prachuab, department's Seismological Bureau, said the department could not give a real-time warning because it did not have the satellite technology to do it.
Thai Prime Minister Thaksin Shinawatra refused to answer reporters' questions Tuesday about tsunami alerts.
But Australian Prime Minister John Howard said he would investigate what role his country could play in setting up an Indian Ocean warning system. One Australian official said at least a year would be needed to set one up.
Scientists said seismic networks in the region recorded Sunday's earthquake, but without wave sensors in oceans that would have tracked the path of the waves, there was just no way to determine the direction a tsunami would travel.
"If they had tidal gauges and a tsunami warning system, many people who died would have been saved," said Waverly Person, director of the U.S. Geological Survey national earthquake information service in Golden, Colo.
"They could have tracked the waves," he said. "They won't tell you how high the waves will be, but they can tell you when they will hit. Local authorities can warn citizens to get off the coast."
Such a system presumes, however, an organized communication system and widely understood procedures and discipline by hotel operators, fishing villages, and local authorities to clear the coastline quickly in case of a coming disaster.
Most of developing Asia lacks such infrastructure, and casualties were by far highest in three highly impoverished areas — the coasts of eastern Sri Lanka and southeastern India, and the northern tip of Indonesia's Sumatra island.
An international warning system in the Pacific was started in 1965, the year after tsunamis associated with a magnitude 9.2 temblor struck Alaska in 1964. It is administered by the U.S-based National Oceanic and Atmospheric Administration.
Member states include all the major Pacific rim nations in North America, Asia and South America, as well as the Pacific islands, Australia and New Zealand.
Tsunamis occur only occasionally, but they are much rarer in the Indian Ocean than the Pacific, where one occurs every few years.
Phil McFadden, chief scientist with the government-funded Geoscience Australia, said that places close to the epicenter of the earthquake would have been hit so quickly that any warning would have come too late.
But if there had been a Pacific-style alert system covering the Indian Ocean, "there would have been time for people in Sri Lanka, across in the Maldives or somewhere like that to have done something about it," he said.