Zambian Mining Minister Kaunda Lembalemba said the toll is expected to rise.
The blast occurred around 11am local time in a workshop at the plant, which supplies explosives to local copper mines.
Rescue teams are working to clear the site in Kitwe, around 300 kilometres north of Lusaka.
“So far, 46 people have been confirmed dead though the figure might be higher than that. The rescue operation is still going on to recover bodies,” Lembalemba told the AFP newswire.
President Levy Mwanawasa expressed shock over the accident and ordered the mines minister to immediately open an investigation to establish the cause.
“I would have loved to go and check for myself what has happened. I’m extremely grieved at the loss of so many lives,” said Mwanawasa, speaking on state television.
Lembalemba told Reuters news agency that the entire plant was razed.
“Most people were burned badly while others were just in pieces,” he said.
“The scene was unpleasant and I shed tears when I saw the pieces of human flesh.”
Copper mining accounts for about 60 percent of foreign earnings in the poor southern African country, which has been struggling since the 1970s following a drop in copper prices.
Chinese and Indian investors have moved in over the past years to buy major stakes in the mines.
The BGRIMM Explosives factory is a major supplier of explosives to Zambia’s copper mines.
It has been operational since October 2003 and has the capacity to produce 6,000 metric tonnes of explosives each year.
Zambia, a former British colony, is one of the world’s poorest countries with close to 64 percent of its 10 million inhabitants living on less than two dollars a day.