Grieving families, attack survivors and representatives from several embassies will attend a memorial in Bali's popular tourist hub of Kuta, where Al-Qaeda-linked militants detonated bombs at a bar and nightclub on October 12, 2002.
"It's okay that some people have forgotten what happened 20 years ago but there are still real victims, there are children who lost their parents in the bombing," 47-year-old victim Thiolina Marpaung, one of the organisers of the memorial who has left with permanent eye injuries in the attack, told AFP.
The candlelight vigil will be held at the site of the attack by victims' family members to mark Southeast Asia's deadliest terrorist attack and remember the 202 victims.
Most were foreign holidaymakers from more than 20 countries but Australia suffered the biggest loss, with 88 dead.
Australia's prime minister told a memorial service in Sydney Wednesday that the horror of the bombings was swiftly countered by incredible acts of self-sacrifice and bravery.
"They sought to create terror, but people ran towards the terror to do what they could for friends and strangers alike," Prime Minister Anthony Albanese told a crowd gathered under light rain at the city's famous Coogee Beach.
Albanese said the Bali bombings had left a permanent mark on Australia's national identity, in a similar fashion to the devastating Gallipoli campaign of World War I.
Some 88 doves were released during the memorial -- one for each Australian killed.
In Bali, the Australian consulate there will also have a memorial service, where Indonesian President Joko Widodo will address families by video and former Australian prime minister John Howard will deliver speeches.