Looking somber and stoic, the 77-year-old Akihito said the problems at Japan's nuclear-power reactors, where authorities are battling to prevent a catastrophe, were unpredictable after an earthquake he described as "unprecedented in scale."
TV stations interrupted coverage to carry the emperor's first public appearance since last week's massive earthquake and tsunami that killed thousands of people.
"I am deeply hurt by the grievous situation in the affected areas. The number of deceased and missing increases by the day and we cannot know how many victims there will be. My hope is that as many people possible are found safe," Akihito said.
"I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times," he said, urging survivors not to "abandon hope."
For elderly Japanese at least, the sudden message from the emperor doubtless called to mind the August 15, 1945, radio broadcast by his father, Emperor Hirohito, announcing the country's surrender in World War Two.
That was the first time the emperor's voice had been heard on radio and his use of formal court language meant most of those listening could not understand what he was saying.