The disaster, which was triggered by the most powerful earthquake in more than 40 years, ravaged Indian Ocean coastlines on 26 December 2004, smashing cities, seaside communities and holiday resorts in a dozen countries from Indonesia to Somalia.
Thousands of foreign survivors and bereaved relatives are expected to head to the affected countries to mark the event and to promote solidarity and remembrance, while the tsunami disaster zone is still struggling with recovery efforts.
Concrete plans needed
Problems relating to the coordination and distribution of relief aid remain, and relief agencies say rebuilding programmes still lack concrete action plans.
According to the United Nations, one year on from the disaster many survivors still live in substandard conditions that fail to meet international criteria for adequate housing.
Women are also exposed to physical and sexual violence in some camps, and to greater domestic violence, according to Miloon Kothari, UN special rapporteur on housing, and Walter Kaelin, the UN secretary-general's special representative on the human rights of internally displaced persons.
They have called on governments in the affected region to provide permanent housing and restore livelihoods in an equitable way.
"We are concerned that a year later, reconstruction efforts are plagued by serious delays and have not been awarded the priority they so urgently warrant," the investigators said.
Tsunami survivors continue to suffer from inequities in aid distribution and sub-standard housing "resulting from political dynamics, bureaucratic inefficiencies and caste affiliation", according to the UN investigators.
Senior European Union officials have also urged better coordination of billions of dollars in international aid, warning that tsunami survivors were still living in dire conditions.
In a first anniversary progress report on reconstruction efforts, the EU said the capacity of both countries' governments remained stretched to the limit.
One year on reconstruction
efforts remain stretched
Benita Ferrero Waldner, the EU External Relations Commissioner, said rebuilding of villages and communities had been too slow, adding that the EU stood ready to provide more urgent aid to people still homeless a year after the disaster.
She also said that coordination between governments, aid agencies and recipients had to be boosted to ensure that generous aid donations from the public and governments were spent wisely.
The EU's report said new houses for victims of the tsunami in Sri Lanka were still not available because of a lack of land, labour and material to build them.
The report also warned Sri Lanka to start disbursing pledged aid before funding deadlines expired.
About $1.9 billion has been pledged for the island nation.
However, Ferrero Waldner said increased violence and tensions between Tamil Tiger guerrillas and Sri Lankan government forces were hampering relief efforts.
She appealed to the new government in Colombo to "re-engage in the peace process" with the separatists.
In Indonesia's northern Aceh province, worst hit by the tidal waves, the EU said reconstruction efforts were making progress.
The EU warned, however, that progress in reconstruction depended on the success of a peace accord between Aceh rebels and the Indonesian government.
The tsunami left more than
216,000 people dead or missing
The impact of the tsunami disaster helped spur an historic peace agreement between the Indonesian government and Acehnese rebels in August this year, but the tragedy did not produce the same result in Sri Lanka, where unrest persists amid the suffering.
The Aceh peace peace accord signed on 15 August in Helsinki, saw the rebels drop their demand for independence and pledge to hand over their arsenal of firearms.
In return, the Indonesian government agreed to pull out all non-local military and police forces by the end of this year, grant amnesty to former fighters and allow them to start political parties.
But it seems the post-tsunami reconciliation felt in Indonesia has not spread to Sri Lanka, where more than 60,000 people have been killed since the start of the ethnic insurgency in 1972.
The South Asian nation's majority Sinhalese community and the minority Tamils are fighting for control of a vast swathe of territory now held by the rebels in the island's north and east.
Peace hopes were initially high in the wake of the tragedy, but quickly began to fade as the Colombo government and the rebel Liberation Tigers of Tamil Eelam (LTTE) fought over how to share millions of dollars in aid funds.
The Tigers recently gave Mahinda Rajapakse's government until the end of the year to come up with a "reasonable" political settlement or risk a war for full independence - a deadline dismissed by the authorities in Colombo.