BANGKOK — As Batik Air Flight 6231 readied for takeoff, the ground began to violently shake in Palu, a city on the eastern Indonesian island of Sulawesi.
In the wobbling air traffic control tower, as other personnel fled, Anthonius Gunawan Agung, 21, stayed put, helping guide the pilot down the runway. Moments after the plane went airborne, the earthquake intensified, and Mr. Agung leapt out of the tower to his death as its roof collapsed.
As the pilot of the flight, Ricosetta Mafella, ascended over the coast, he noticed what he called on Instagram a “strange wave” — a sign of the devastating tsunami that would soon sweep over Palu and other parts of Central Sulawesi Province.
The twin disasters — a 7.5-magnitude earthquake, and the swirling wall of water it unleashed — killed at least 405 people in Palu and destroyed thousands of buildings there, including a shopping mall, a hotel, seaside restaurants and several mosques.
“We have found corpses from the earthquake as well as bodies swept up by the tsunami,” Sutopo Purwo Nugroho, a spokesman for the Indonesian disaster agency, said in a television interview.
Indonesian officials were preparing for a sharp rise in the death toll because search-and-rescue teams had yet to reach populous coastal settlements near Palu. Vice President Jusuf Kalla of Indonesia told a local news website that thousands may have died, with an unknown number washed out to sea.
The tsunami inundated Palu just as preparations were underway for a beachside festival with dances and other performances, and the festival’s security personnel are believed to be among the victims.
Although Indonesia is chronically at risk of tsunamis, Andri Manganti, a resident of Palu who lost his home in Friday’s earthquake, said that no warning siren sounded before the tidal wave — estimated to be a towering 18 feet high — struck the city of about 300,000 people.
Text messages that were supposed to warn locals of the possibility of a tsunami did not go out as planned because cellphone towers had been downed by the earthquake, Mr. Sutopo said.
Indonesia’s meteorological and geophysics agency is facing criticism for having lifted its tsunami warning little more than half an hour after the earthquake struck. It is not yet certain whether the devastating wave that was described by Mr. Sutopo as around 18 feet in height struck before or after the tsunami warning was lifted.
Mr. Sutopo said on Saturday that as he was preparing information to alert the public about the tsunami threat, the warning was abruptly halted by the geophysics agency.
Video clips taken along a bustling seaside avenue in Palu appear to show two successive waves, battering the coast within a few minutes of each other.
A video taken from the seaside Palu Grand Mall shows the moments just before the tsunami made landfall and then its destructive force as the first wave swept over the beach, pushing cafes off the sand and into the street, where cars and pedestrians were soon submerged.
In the clip, taken from an elevated parking garage just off the beach, a man can be heard yelling “tsunami” and calling out to God as he tries to warn those walking by on the road below to get to higher ground. On the horizon, the second wave is already visible.
Another video, taken just down the road, shows the second wave crashing over the roofs of one-story buildings, which then disappear beneath the turbulent water. The water surged around a mosque, whose large green dome had already collapsed, probably from the quake.
On Saturday afternoon, Mr. Sutopo said that in addition to the 405 people confirmed dead in Palu, 540 people were injured and 29 were missing. About 2.4 million people are believed to have been affected by the earthquake, according to Indonesia’s central statistics agency.
Mr. Manganti’s aunt and cousin, who lived by the coast, are among those yet to be found. His sister, Lidia Manganti, who lives in Jakarta, has been frantically trying to reach her relatives over phone lines that are working only sporadically.
“I’m so worried that I could not sleep the whole night,” she said. “I’m afraid that the chances for survival for my aunt and cousin are small, but I’m hoping that it’s just a problem with telecommunications.”
Indonesia’s air navigation agency, which posted a series of messages to Twitter describing the moments at the airport during the earthquake, said it had posthumously promoted Mr. Agung, calling him one of its “warriors.” Mr. Mafella, the pilot, called him a “guardian angel” on Instagram.
With so little information trickling out from areas outside the city, and based on Indonesia’s tragic history with tsunamis, fears have multiplied that the final death toll could grow far worse.
Mr. Kalla, Indonesia’s vice president, noted that when the Indian Ocean tsunami struck on Dec. 26, 2004, the death toll recorded that night in Aceh, on the northern tip of the Indonesian island of Sumatra, was around 40 people. The eventual body count in Aceh exceeded 130,000.
Indonesia, an archipelagic nation of more than 13,000 islands, is one of the most seismically active places on earth and is regularly pummeled by natural disasters.
The 2004 Indian Ocean tsunami, which was triggered by a magnitude 9.1 earthquake and killed most of its victims in Aceh, claimed around 230,000 lives in 14 countries. It ranks as one of the world’s most devastating natural disasters.
Last month, a magnitude 6.9 earthquake hit Lombok and the Gili islands southwest of Sulawesi, killing more than 460 people.
Since the 2004 tsunami, Indonesia has strengthened its disaster response. A new disaster management agency was created, with provincial units that can report quickly to the center.
“It’s quite a difference from Aceh times,” said Selina Sumbung, Save the Children’s implementing partner in Indonesia. “Coordination is a lot better, and we find out information a lot faster.”
But obstacles remain. Provincial authorities in Indonesia, who hold significant power, have had a habit of turning away offers of help from outsiders, say the staff of international charities.
In the aftermath of the Lombok earthquake, for instance, foreign nongovernmental organizations were told they were not needed. Even though more than 10 percent of Lombok’s population had been dislocated, no national disaster was declared, a prerequisite for catalyzing international aid.
“In many cases, unfortunately, they’ve been very clear that they’re not requesting international assistance, so it’s a bit challenging,” Ms. Sumbung said. While Save the Children is putting together a team to travel to Palu, it is not yet sure whether foreign staff can work on the ground.
Mr. Sutopo, the national disaster agency spokesman, said Indonesian officials were assessing the situation in Palu to see whether international agencies would be allowed to contribute to the aid effort.
Given the earth shaking that Indonesia constantly endures, the country remains woefully underprepared for nature’s wrath. While tsunami shelters have been built in Aceh, they are not a common sight on other coastlines. The apparent lack of a tsunami warning siren in Palu, even though a warning had been in effect, is likely to have contributed to the loss of life.
At the best of times, traveling between Indonesia’s many islands is challenging. Natural disasters make logistics even more complicated. A hospital ship that had been stationed in Lombok to treat earthquake victims is making its way to Palu, but it will take at least three days to reach the site of the new calamity.
President Joko Widodo made improving Indonesia’s tattered infrastructure a centerpiece of his election campaign, and he has lavished money on roads and railways. But funding shortfalls have plagued Mr. Joko’s administration as he faces re-election next year.
Mr. Joko is also facing pressure from lingering sectarian tensions in Indonesia, where members of the Muslim majority have embraced a more conservative form of the faith.
Palu, the city devastated by the quake and tsunami on Friday, is the capital of Central Sulawesi Province. The island of Sulawesi has been divided, at times bloodily, between Muslim and Christian populations. While Indonesia is the world’s largest Muslim-majority nation, it also has a significant number of Christians.
In the late 1990s and early 2000s, there was widespread communal violence in and around Poso, a port city not far from Palu that is mostly Christian. More than 1,000 people were killed and tens of thousands dislocated from their homes as Christian and Muslim gangs battled on the streets, using machetes, bows and arrows, and other crude weapons.
Hannah Beech and Richard C. Paddock reported from Bangkok, and Muktita Suhartono from Penang, Malaysia. Megan Specia contributed reporting from New York.