A chorus of voices singing the Indonesian national anthem marked the start of the ceremony at a 8ha park in Indonesia's Banda Aceh - the main city of the province closest to the epicentre of the massive quake and which bore the brunt of the destructive waves.

"Thousands of corpses were sprawled in this field," Indonesian Vice-President Jusuf Kalla told the crowd of several thousand - many among them weeping.

"Tears that fell at that time... there were feelings of confusion, shock, sorrow, fear and suffering. We prayed. And then we rose and received help in an extraordinary way. Help came from Indonesia and everyone else, our spirits were revived," he said, hailing the outpouring of aid from local and foreign donors.

"We are gathered here today to remember the historic disaster that took place on Dec 26. As we know, it was one of the biggest to have ever happened on our Earth," Aceh Governor Zaini Abdullah told the crowd of several thousand including dignitaries gathered for the official memorial.

"The disaster was also an awakening - to be aware of our environment and to continue to be vigilant and understand how to deal with disasters," he said.

"Learning from our experience, we call for strengthening of solidarity in handling disasters to lighten the load of disaster victims across the world," he added, hailing the outpouring of aid from local and foreign donors.

Mosques also held prayers across the province early on Friday while people visited the mass graves - the resting place of many of Indonesia's 170,000 tsunami dead.

But a Red Cross display of hundreds of salvaged ID documents and bank cards, also served as grim reminder that many victims simply vanished.

Muslim clerics, tsunami survivors and rescue workers led around 7,000 mourners gathered at Banda Aceh's black-domed Baiturrahman Grand Mosque for memorial prayers late on Thursday.

Malaysian cleric Syeikh Ismail Kassim said he and several hundred compatriots attended to show support for Aceh.

"We hope Aceh people will not waver as a result of the calamity that has befallen them," he told AFP.

Mr Zaini Abdullah thanked Indonesians and the international community in his address at the mosque, one of the few buildings which withstood the wrath of the massive earthquake and ensuing waves which left 170,000 people in the country dead or missing.

"The tsunami had caused deep sorrow to Aceh residents from having lost their loved ones," he said.

"Sympathy from Indonesians and the international community has helped (Aceh) to recover," he added.

He also called on residents not to "dwell in our grief, so that we could rise from adversity and achieve a better Aceh".

Mr Kamaruddin, a fisherman who like many Indonesians goes by one name, said he attended the prayers to remember his wife and three children who died in the tsunami.

"I hope there will be no more disasters in Aceh," the 50-year-old said.

Tsunami anniversary committee chairman Azhari Hasan told AFP: "Prayers are being held in memory of the victims who perished or went missing.

"We also hope that survivors - and the families they left behind - can stay strong and be aware of the (future) threat of disasters," he added.

In Meulaboh, a fishing town considered to be the ground zero of the tsunami - where 35m-high waves flattened almost everything - Indonesian flags were flown at half-mast as small groups of residents held night prayers at mosques.

The main memorials were planned for Friday morning, starting in Aceh, which was hit first by the waves, then moving to Thailand where candlelit ceremonies are expected in the resort hubs of Phuket and Khao Lak.

In southern Thailand, where half of the 5,300 dead were foreign tourists, a smattering of holidaymakers gathered at a memorial park in the small fishing village of Ban Nam Khem, in a reminder of the global scale of the disaster.

As the ceremony began, survivors recounted stories of horror and miraculous survival as the churning waters, laden with the debris of eviscerated bungalows, cars and boats, swept in without warning, killing half of the village's inhabitants.

Swiss national Raymond Moor said he noticed something was amiss when he saw a white line on the horizon rushing towards the beach where he and his wife were having breakfast.

"I told my wife to run for her life... it wasn't a wave but a black wall," he said, describing being caught up in the water moments later like "being in a washing machine".

"A Thai woman from the hotel saved my life by pulling me up to a balcony. She died later," he said, breaking into tears.

"Everyone knew someone affected by the tsunami, I knew people too. We want to show our respect," said Mr Agnes Moberg, 18, from Sweden, which lost more than 500 of its nationals and was due to honour its dead later Friday.

Nearby, Ms Somjai Somboon, 40, said she was yet to get over the loss of her two sons, who were ripped from their house when the waves cut into Thailand.

"I remember them every day," she told AFP, with tears in her eyes.

Among the international commemorations, in Sweden, which lost 543 to the waves, the royal family and relatives of those who died will attend a memorial service in Uppsala Cathedral on Friday afternoon.

In Sri Lanka, where 31,000 people perished, preparations were underway to hold a memorial at a railway site where waves crashed into a passenger train, killing 1,500 people.

Ahead of the ceremony, a train guard who survived told AFP a lack of knowledge of tsunamis - in a region which had not experienced one in living memory - led to more deaths than necessary.

"We had about 15 minutes to move the passengers to safety. I could have done it. We had the time, but not the knowledge," 58-year-old Mr Wanigaratne Karunatilleke said.

Mr Karunatilleke was among the few survivors of the disaster on the Ocean Queen Express, which has become a symbol of the disaster in Sri Lanka, and will be at the centre of commemorations for the country's 31,000 victims on Friday.

Mr Karunatilleke, who as head guard was responsible for passengers' safety, overruled the signalling system after the first wave and ordered the driver to move, but by then it was too late.

When the tsunami hit, he became trapped inside a compartment that was floating in the water, managing to escape through a window.

Many of the tsunami's victims died in dark, churning waters laden with uprooted trees, boats, cars and eviscerated beach bungalows, as the waves surged miles inland and then retreated, sucking many more into the sea.

Thailand saw 5,395 people killed by the disaster - half of them foreign holidaymakers.

British survivor Andy Chaggar was in a bungalow on Thailand's Khao Lak when the tsunami waves struck, taking his girlfriend's life and sweeping him inland.

"I came to in the water... there was glass, metal, there were pieces of wood, bricks, it was like being in a washing machine full of nails," he told AFP on Thursday, on the same beach where he lost his girlfriend.

There will also be events in several European capitals to remember foreign nationals who perished.

As the scale of the tragedy emerged, disaster-stricken nations struggled to mobilise a relief effort, leaving bloated bodies to pile up under the tropical sun or in makeshift morgues.

The world poured money and expertise into the relief and reconstruction, with more than US$13.5 billion (S$17.9 billion) collected in the months after the disaster, representing more than US$7,100 for each person affected by the tsunami.

Almost US$7 billion in aid went into rebuilding more than 140,000 houses across Aceh, thousands of kilometres of roads, and new schools and hospitals.

The vast majority of Indonesia's 170,000 victims perished in the province, among them tens of thousands of children.

But the disaster also ended a decades-long separatist conflict, with a peace deal between rebels and Jakarta struck less than a year later.

It also prompted the establishment of a pan-ocean tsunami warning system, made up of sea gauges and buoys, while individual countries have invested heavily in disaster preparedness.

But experts have cautioned against the perils of "disaster amnesia" creeping into communities vulnerable to natural disasters.

What happened that day

On Sunday, Dec 26, 2004 at 7:58am local time, a massive earthquake with a magnitude of 9.3 struck off Indonesia, unleashing a devastating tsunami.

The strength of the quake - the biggest in the world since 1964 - was such that the Earth shifted - unleashing a multi-metre wave that 30 minutes later devastated the Indonesian province of Aceh, to the north of Sumatra.

Aceh was the most affected region: dozens of villages were wiped from the map and the strength of the tsunami went so far as to shift the islands. An estimated 131,000 died on the west coast of Sumatra.

The wave also swept the whole of the Indian Ocean's shoreline, hitting the coasts of Sri Lanka, India - especially the Andaman and Nicobar islands - Thailand, Myanmar, Malaysia, the Maldives and Bangladesh.

Around six hours after the start of the disaster the coasts of East Africa - Somalia, Tanzania, Kenya - were reached by the tidal wave.

In the space of several hours at least 220,000 people died, of which nearly 170,000 were in Indonesia, 31,000 in Sri Lanka, 16,400 in India, and 5,400 in Thailand, according to an official count.

Two hundred people were killed in other Asian countries hit by the wave, while 300 perished in East Africa.

The entire international community was affected by the disaster. Out of the 5,400 casualties in Thailand, nearly half were foreigners representing 37 nationalities.

European countries, including Sweden (543 dead), Germany (537), Finland (180), Britain (150), Switzerland (110), France (95), Denmark (50) and Norway (80), lost 1,700 people, mainly tourists seeking Christmas sun.

The deadly waves particularly hit the young, but several thousand children found themselves orphans too, while tens of thousands suffered from psychological problems.

There was countless material damage and more than one million people were left homeless.

The tsunami ravaged the Indian Ocean coastline's ecosystem, including Aceh's mangroves and Thailand's coral reef, and unleashed chemical pollution.