Japan evacuated more than 320,000 people today as a tsunami triggered by Chile's massive earthquake sent waves up to 1.20 metres (four feet) high barrelling into its long Pacific coastline.
Seawater swells inundated buildings in several harbours. Authorities warned coastal communities to stay on high alert and keep clear of the shore as more powerful tsunami waves could follow.
There were no immediate reports of injuries or major property damage.
Japan's "major tsunami alert" -- warning of the threat of monster waves that could top three metres -- was the country's first in more than 15 years.
From early morning, tsunami warnings flashed across all television channels and evacuation sirens wailed across the east coast of the archipelago.
Massive steel gates slammed shut across the entrances to fishing ports. Coastguard vessels and air force jets fanned out to search for stray ships still at sea and to observe any damage.
"Please do not approach the coast at any cost," Prime Minister Yukio Hatoyama said in nationally televised comments. "We should not drop our guard. I would like people to take all possible measures" to stay safe, he said.
East-coast railway services were halted and cities and towns cancelled festivals and local elections, while public broadcaster NHK repeatedly warned local residents not to go near beaches or a river mouth.
More than 320,000 people were ordered to evacuate to schools and other public buildings in the Aomori, Iwate and Miyagi prefectures on the main island of Honshu, according to a Kyodo news agency tally.
When the tsunami came, it was weaker than feared. The initial wave, just 30 centimetres (one foot) high, hit the remote port of Nemuro on the northern island of Hokkaido early in the afternoon.
The water in Nemuro quickly rose to flood port-side warehouses and leave parked vehicles stranded.
"The water level is seen repeatedly rising and falling," said an NHK reporter on the scene. "Ships are evacuating the port."
The fluctuating water level meant "we are seeing a typical tsunami phenomenon", Yoshinobu Tsuji, associate professor at the Earthquake Research Institute at the University of Tokyo, told the public broadcaster.
The highest tsunami wave of 1.20 metres hit the port of Kuji in Iwate prefecture, where almost 9,000 people were evacuated.
"At one point, water rose to cover some parts of the port," said a city official. "It is going up a little and down a little.
"We can still see waves coming. We are not yet in the situation where we can lift the warnings and advisories. We still need to monitor the conditions."
Saturday's quake in Chile, which killed at least 300 people, revived raw memories for Japan.
In 1960, a 9.5-magnitude earthquake in Chile -- the largest on record -- sent a tsunami hurtling across the Pacific, leaving more than 140 people dead in Japan.
However, construction standards and safety drills have vastly improved and Japan is today a world leader in disaster preparedness.
"Last time (in 1960), waves that hit after the first one became even more powerful," said Yasuo Sekita, a Meteorological Agency official in charge of monitoring earthquakes and tsunamis.
On Saturday, Okinawa was hit by a big 7.0-magnitude quake, triggering minor tsunamis but causing only small damage to housing and minor injuries.
In 1923, the Great Kanto Earthquake in the Tokyo area claimed more than 140,000 lives, many of them in fires. In 1995, a major quake killed some 6,400 people in Kobe and other western Japanese cities.