Millions of Japanese people awoke to ominous text messages urging them to take cover after North Korea fired a missile over the country.
TV and radio stations also interrupted regular programming to broadcast the "J-Alert", while bullet trains were temporarily halted as warnings went out over loudspeakers in towns in the northern island of Hokkaido.
The ballistic missile, fired at 5.58am local time, fell into the sea 735 miles (1,180 km) east of Cape Erimo.
Shortly after the launch millions of people received mobile phone messages sent by the Japanese government.
"A missile was fired from North Korea. Please evacuate to a sturdy building or basement," the text read.
A second message sent minutes later said: "A North Korean missile passed over this area. If you find suspicious things, please never touch them."
The J-Alert was issued in 12 prefectures, including Hokkaido, reportedly making it the most widespread warning issued since the system began operating in 2007.
But some recipients were confused about how to respond and questioned whether it allowed them enough time to react.
"I was woken by the missile alert on my cellphone," said Ayaka Nishijima, 41, an office worker from Morioka, the capital of Iwate prefecture, 300 km (180 miles) south of Cape Erimo.
"I didn't feel prepared at all. Even if we get these alerts there's nowhere to run. It's not like we have a basement or bomb shelter, all we can do is get away from the window," she told Reuters.
The J-Alert was the third issued about a North Korean missile launch since 2007, reported the Asahi Shimbun.
Japan did not shoot down the missile, thought to have been a recently developed intermediate-range Hwasong-12, which travelled 2,700km and reached a maximum height of 550km.
North Korea has conducted dozens of ballistic missile tests under leader Kim Jong-un, the most recent on Saturday, but firing projectiles over mainland Japan is rare.
"North Korea's reckless action is an unprecedented, serious and grave threat to our nation," Japanese Prime Minister Shinzo Abe said.
The United Nations Security Council would meet later on Tuesday to discuss the test, diplomats said.
Earlier this month, the 15-member Security Council unanimously imposed new sanctions on North Korea in response to two long-range missile launches in July.
