SAMUT SAKORN, Thailand (Reuters) - About 200 chanting Buddhist monks and laymen prayed and walked around piles of human skulls and other bones on Thursday in sacred preparation for a mass cremation next week. The hundreds of skulls were among 21,347 bones of unclaimed bodies that were exhumed from a decades-old cemetery in downtown Bangkok that has been overrun by an expressway. The mass cremation of the bones -- believed to be the world's largest -- will take place on Monday. The Thai, Chinese and Buddhist monks and worshippers in yellow, white and black robes were praying for the second day of a week-long effort to ensure the spirits of the dead bodies can go to heaven. For Buddhists, particularly those of Chinese descent, this is the best way for people to earn merit and ensure their own spirit's passage to heaven. In the past, all of the bodies and bones that went unclaimed at the cemetery were exhumed about two years after their burial and cremated in Buddhist tradition. But when the expressway approached, the owners decided to relocate the entire cemetery to a new plot of land in this town on the outskirts of Bangkok and cremate all unclaimed bodies and bones. The cemetery was owned by a private foundation, Poh Tek Tun, which buries the bodies, then cleans and cremates unclaimed bones on an annual basis. If the bodies haven't been claimed after two years, the foundation organises their cremation. The move to Samut Sakorn was expected to help the foundation in its efforts.