BANDA ACEH, Indonesia — People gathered in prayer and visited mass graves in Indonesia’s Aceh province Thursday to mark 20 years since a massive Indian Ocean tsunami hit the region in one of modern history’s worst natural disasters.

Many wept as they placed flowers at a mass grave in Ulee Lheue village, where more than 14,000 unidentified tsunami victims are buried. It is one of several mass graves in Banda Aceh, the capital of Indonesia’s northernmost province, which was one of the areas worst hit by a magnitude 9.1 earthquake and the massive tsunami it triggered.

“We miss them and we still don’t know where they are. All we know is that every year we visit the mass grave in Ulee Lheue and Siron,” said Muhamad Amirudin, who lost two of his children 20 years ago and never found their bodies.

“This life is only temporary, so we do our best to be useful to others,” Amirudin, visiting the grave with his wife, said.

The powerful earthquake off the coast of the Indonesian island of Sumatra on Dec. 26, 2004, triggered a tsunami that killed around 230,000 people across a dozen countries, reaching as far as East Africa. About 1.7 million people were displaced, mostly in the four worst-affected countries: Indonesia, Sri Lanka, India and Thailand.

More than 170,000 people died in Indonesia alone.

Even though 20 years have passed, survivors in Indonesia are still grieving the loved ones they lost to the giant wave that flattened buildings all the way to the provincial capital, Banda Aceh.

Hundreds of people gathered to pray at the Baiturrahman Mosque in downtown Banda Aceh. Sirens sounded across the city for three minutes to mark the time of the earthquake.

Infrastructure in Aceh has been rebuilt and is now more resilient than before the tsunami struck. Early warning systems have been installed in coastal areas to alert residents of incoming tsunamis, providing crucial time to seek safety.

The rebuilding efforts were made possible by the support of international donors and organizations, which contributed significant funds to help the region recover. Schools, hospitals and essential infrastructure destroyed in the disaster have been rebuilt.

In Thailand, people gathered at a memorial ceremony in Ban Nam Khem, a small fishing village in Phang Nga province that bore the brunt of the devastating wave in the country.

The tsunami claimed the lives of more than 8,000 people in Thailand, leaving a deep scar in the nation’s history. Nearly 400 bodies remain unclaimed.

Mourners shed tears and comforted each other as they laid flowers at the village’s tsunami memorial. Around 300 people joined a modest ceremony with Muslim, Christian and Buddhist prayers.

In India, hundreds gathered at Marina beach in the southern city of Chennai in Tamil Nadu state. They poured milk into the sea to propitiate gods and offered flowers and prayers for the dead as drums beat in the background.

According to official data, 10,749 people were killed in India, nearly 7,000 of them in Tamil Nadu alone.

In Sri Lanka, survivors and relatives of tsunami victims gathered at the coastal village of Pereliya and laid flowers at a memorial that commemorates nearly 2,000 passengers who died when their train, the Queen of the Sea, was hit by the wave. Only a few dozen people are believed to have survived.

Anura Ranjith joined the mourners to pay respects to his younger sister, Anula Ranjani, and her 9-year-old daughter, who were passengers on the train.

“I looked for them everywhere for years and still, no information about them. Their loss is a great sorrow and pain for me. I am still grieving,” he said.

More than 35,000 people died in Sri Lanka in the tsunami. People across the country observed two minutes of silence Thursday in memory of those who lost their lives.