Indonesian officials say at least 384 people died and 570 were injured following the 7.5-magnitude earthquake Friday and a subsequent tsunami that hit two cities - Palu and Donggala - in central Sulawesi province.
Indonesian Vice President Jusuf Kalla said the death toll could climb into the thousands.
Authorities said Saturday hundreds of people were on the beach in Palu for a festival when the earthquake and tsunami struck, sweeping many people away to their deaths in the giant waves.
"When the threat arose ... people were still doing their activities on the beach and did not immediately run and they became victims," Indonesian National Board for Disaster Management spokesman Sutopo Purwo Nugroho said at a media briefing in Jakarta. "The tsunami didn't come by itself, it dragged cars, logs, houses. It hit everything on land."
Nugroho said all of the fatalities occurred in Palu and that information from the heavily damaged cities of Donggala and Mamuju was still not available Saturday due to damaged roads and communications disruptions.
Nearly 17,000 people were evacuated to 24 shelters in Palu, which was rocked with aftershocks through Saturday afternoon. An undetermined number of people were also evacuated to the neighboring city of Makassar.
Nugroho said the tsunami hit with a speed of 800 kilometers per hour, destroying buildings and infrastructure. He said thousands of houses, hospitals, shopping malls and hotels collapsed and a landslide has cut off Palu's main highway.
A large bridge spanning a coastal river in Sulawesi province collapsed.
Nugroho said search and rescue teams have been dispatched.
Television footage showed people being treated outdoors in makeshift medical facilities, while bodies, some of them in bags, are lined up on the streets.
An Indonesia emergency official said, "Bodies of victims were found in several places because they were hit by the rubble of collapsing buildings or swept by tsunami."
Power and telecommunications lines have been knocked out, hampering the rescue operations Saturday.
Palu's airport was reopened for relief efforts but will remain closed for commercial flights until October 4, Nugroho said. The military is transporting aid in cargo planes from Jakarta and other cities to help evacuees who are dire need of food and other necessities.
U.N. spokesman Stephane Dujarric said U.N. officials were in contact with Indonesian authorities and “ready to provide support as required.”
President Joko Widodo will visit Palu on Sunday to observe search and rescue efforts by the military and government agencies.
VOA correspondent Yoanes Litha was in the small town of Parigi Saturday night after his trek by motorcycle to nearby Palu was delayed by more than 10 hours because all roads and bridges were either damaged or blocked by landslides.
Indonesia is one of the most disaster-prone nations on Earth. It sits on the so-called "Ring of Fire," an arc of volcanoes and fault lines in the Pacific Basin.