On December 26, 2004 a 9.3-magnitude earthquake off Indonesia's western coast sparked a series of towering waves that wrought destruction across countries as far apart as Indonesia, Thailand, Sri Lanka and Somalia.
Among the victims were thousands of foreign holidaymakers enjoying Christmas on the region's sun-kissed beaches, striking tragedy into homes around the world.
Tsunami survivors and rescue workers will lead several thousand people in a gathering at Banda Aceh's black-domed Baiturrahman Grand Mosque for memorial prayers late Thursday.
"Prayers are being held in memory of the victims who perished or went missing," tsunami anniversary committee chairman Azhari Hasan told AFP.
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"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 35 metre-high waves flattened almost everything -- Indonesian flags were flown at half-mast as residents prepared to hold night prayers.
There will also be events in Sri Lanka, including at the site where a train carrying 1,500 people was washed away, as well as in several European capitals to honour foreign nationals who perished.