Prayers for the 37 people killed, including 23 children, at a preschool in northern Thailand began on Saturday..
Grieving families gathered at Buddhist temples to make offerings to the spirits of the deceased toddlers and adults.
The ceremony will be held for three days ahead of cremations and burials on Tuesday.
It follows a rampage by a former police sergeant who shot and slashed his victims with knives on Thursday.
The 34-year-old officer was fired earlier in the year because of drug use, police said.
When he left the daycare centre in the Nong Bua Lamphu province, the ex-policeman drove home and killed his wife and son, and then took his own life.
The mass killing was one of the deadliest in Thai history.
More than 90 children usually attend the preschool, but just over 20 were present on Thursday due to bad weather and problems with the school bus.
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