In Bang Nieng, hundreds lined up on Monday to sign books of remembrance in the shadow of a police patrol boat that was washed one kilometre inland by the waves and now stands as a memorial to the catastrophe.
Many of the messages were brief, with one reading: "We miss you every day", while another simply asked "Why?!"
Swedish couple Mari Olsson and Michael Sanden were among the crowd who were laying flowers and photos of the dead at an alter.
The pair had come back to Thailand with their two surviving daughters to remember a third who was killed when the entire family was caught up in the watery crush.
Olsson said: "We didn't want to be home at Christmas, we didn't want to celebrate."
Somkid Jatusripitak, the Thai deputy prime minister and commerce minister, told the gathering: "The tsunami has touched the lives of countless people here and abroad."
"One year has passed, we continue the rebuilding process. Much progress has been made but we have more to do still."
Elsewhere, other ceremonies, many organised by foreigners, took place on beaches where about 5400 Thais and tourists died when the tsunami struck last December.
On Phuket's Patong beach, Australian Ingrid Hastie threw flowers into the sea in memory of her mother, who drowned in her first-floor hotel room, and declared she would never return to the Thai holiday island.
A Thai woman prays for family
members who died
But Hastie, one of several dozen Australians commemorating their 23 compatriots who died in the waCommves, said she was glad that she came back for the anniversary and thought she could now move on with her life.
"This was the hurdle, I think we're over it," she said.
The mourners threw red roses and purple Thai orchids into the sea, embracing each other as the waves carried the petals back to the shore, while wreaths were laid beneath an Australian national flag erected in the sand.
But as the media scrum pressed into the crowd, a New Zealand woman shouted at journalists to back away as hundreds of onlookers applauded.
"I think this is very rude of you. I know people have left because they have no room. Could the police do their jobs and keep these people back," she said, her voice breaking.
Moments later, thousands of beach-goers stopped to observe a moment of silence.
On Phi Phi island, nearly 10,000 people gathered to lay orchids and garlands of flowers at a tsunami altar, including Apichart Mukda, a Thai diving instructor who broke his leg in the disaster.
Speaking from a wheelchair Apichart said: "The tsunami is my worst memory."
In the afternoon, Thaksin Shinawatra, the Thai prime minister, is to lay a foundation stone for a tsunami memorial in Khao Lak, one of the regions hardest hit by the catastrophe.
At dusk, the prime minister is to preside over an inter-faith service, where Buddhist, Muslim, Hindu, Jewish, Christian and Sikh prayers will be read.
Later in the evening, two young survivors of the tsunami, 11-year-old Briton Tilly Smith and 10-year-old Thai Patiwat Komkla, will read poems before mourners light hundreds of floating lanterns in Khao Lak.
"I think this is very rude of you. I know people have left because they have no room. Could the police do their jobs and keep these people back"
A New Zealand woman,
shouting at journalists
The day-long memorials follow a weekend of services including candlelight vigils, simple prayers and floral tributes.
The physical damage caused by the tsunami there has largely been repaired. Beaches that were littered with debris have been cleaned and rebuilt with memorial parks, tsunami warning towers and protective seawalls.
Thaksin said the government still had 100 million baht ($2.5 million) in tsunami aid left to use, adding that he thought Thailand's tourism numbers, which plummeted after the catastrophe, would recover by next year.