The six young children had just shared snacks bought from a corner store when they began convulsing. The children, all of them under 8, died moments later, adding more victims to a wave of food poisoning that the authorities say has killed nearly two dozen children in a few months.

The South African government declared the poisonings a national disaster on Thursday, taking action after President Cyril Ramaphosa laid out the scale of the danger. At least 890 people have fallen sick, many of them children, he said in a televised address last week, adding that the cause was believed to be a pesticide used by business owners and vendors to fight a rat infestation in neglected townships. Expired and counterfeit food products have also been blamed by grieving family members and some residents.

The size of the outbreak, with deaths reported in provinces across the country, has forced South Africa’s leaders to reckon with the everyday consequences of dysfunctional government departments that are tasked with overseeing food safety, waste disposal, and small business regulations.

The government declared the emergency in a news conference held by half a dozen cabinet ministers, representing portfolios from health and education to agriculture and trade. Officials have also fanned out to inspect stores and to visit mourning families in townships where angry residents have turned on shop owners, many of whom are immigrants.

“These products are just as likely to be sold in shops owned by South Africans,” Mr. Ramaphosa said during his address, trying to curb the anger in a country where violence has in the past flared up between South Africans and migrants from other African countries and South Asia.

After the deaths of the six children in Johannesburg last month, South Africa’s National Institute for Communicable Diseases found traces of terbufos, a hazardous pesticide used in agriculture, in the contents and on the packaging of a snack found with one of the children, Mr. Ramaphosa said. Terbufos, a colorless or pale yellow liquid used on crops, can be fatal if ingested or inhaled, or if it comes in contact with humans, according to the National Institutes of Health.

In other cases, South African health authorities found evidence of aldicarb, an agricultural pesticide that is highly toxic to humans. The pesticide has been banned in South Africa since 2016, Mr. Ramaphosa said.

These highly toxic chemicals had been adopted as a “street pesticide,” he said, to fight a growing rat infestation in South Africa’s formerly segregated townships and mushrooming shanty towns. In poor communities, where municipalities fail to regularly collect waste, business owners had turned to the toxins to keep vermin away.

In yet other cases, expired food products have been blamed as the cause of death. Some residents and outraged families of children who died have, stoked by longstanding anti-immigrant sentiments, blamed foreign owners of corner stores for the poisonings. The owners, they claim, use pesticide to kill rats and sell expired food items or counterfeit brands of processed food to poor communities where people cannot afford to shop in supermarkets.

The stores, known as spaza shops, are often built in a backyard and operated by migrants. In response, the government will now register these shops, Mr. Ramaphosa said. The measures are likely to offer little comfort to the families around the country who have buried young children. In Kimberley, a city in South Africa’s Northern Cape Province, a 4-year-old girl died after eating bread last month. In another province, the Eastern Cape, a 9-year-old girl died after reportedly eating a packet of crisps.

In Johannesburg this week, a 5-year-old boy reportedly died just 20 minutes after eating a snack. His home, in the city’s Soweto township, was not far from where the six friends died just weeks earlier.

“Because these children are friends, they share everything,” Triphina Msimango, who lost her grandson, told the South African Broadcasting Corporation last month. “They shared the snacks among themselves, not knowing they were eating poison.”