It said no miners arrived for morning work aboard buses that normally carry upwards of 1,200 workers per shift to the vast deposit, which accounted for about 6 percent of global production in 2015.
The union said it was prepared for a lengthy strike and that it had resources for 60 days. Escondida's processing plants, which had begun going offline on Wednesday, have completely stopped, the union said.
"If someone suddenly walked into your kitchen and took away your refrigerator, would you just stand there, arms crossed?" asked miner Jose Alcaino, after returning from the night shift.
"That's what's happening here. They want to take away our benefits, our money, they want to work us more."
BHP declined to comment on Thursday.