The biggest earthquake in 40 years hit southern Asia and triggered a massive wall of water that raced across the Indian Ocean, bringing devastation and death to Sri Lanka, India, Thailand and Indonesia. More than 7000 are believed killed.
In countries stretching from the Horn of Africa, down to Tanzania and out into the Indian Ocean, lowland flooding and irregularly fast tidal changes were reported.
In the Seychelles, low-lying coastal roads were flooded by a two-metre surge and power was knocked out at hundreds of homes.
At the airport, fire brigades were forced to wash dozens of fish off the runway each time high tides sent water crashing on to the airfield.
Residents and tourists were warned to be ready to move to higher ground if need be.
Beaches at tourist resorts around Kenya's coastal town of Mombasa were closed because of the extreme tidal movements set off by the earthquake.
"We have had four high tides and four low tides in the last six hours," a spokesman for the Turtle Bay resort in Malindi said, adding that the resort would remain shut until the water calmed.
"We are monitoring the situation very closely, but we don't expect it [the tsunamis] to reach this far," Mombasa provincial police office Alex Rono said.
"I do not know what to expect from the ocean"
In Tanzania people reported unusual sea movements. One beachgoer told the BBC that he saw water rapidly receding from the shoreline at about 12:45.
"The water receded almost a kilometre into the ocean. We were out in the sea at that time and we were dragged back into the ocean with a tremendous force," said Abhay Mavalankar from the capital Dar es-Salaam.
"There was an extremely powerful undercurrent. After some time there were series of huge waves and again the water came swelling back engulfing many people."
In Somalia, strong waves and winds capsized boats and kept fishermen in port.
"I do not know what to expect from the ocean," a Somali fisherman in the coastal town of Adale, 56km north of Mogadishu, said.
Mauritius seemed largely untouched by the post-quake surge, although minor flooding occurred on one of the outlying islands in the archipelago, Rodrigues.
In the Comoros Islands, residents had seen no ill effects.