The current level of political solidarity on the continent, he said, was "almost unprecedented."

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The riots, and the retaliatory measures, could not come at a worse time for regional cooperation. This week, African leaders are meeting in Cape Town, South Africa, to discuss the African Continental Free Trade Area, an agreement made this year that sets the stage for the creation of the largest free-trade area in the world. It would join Africa's more than 1 billion consumers into a single market.

The conflict, while not likely to imperil the free trade agreement, could at least slow its implementation, which is expected to take years, African analysts said.

Nigeria's government, angry that its citizens have been victimised in the South African riots, has pulled out of the Cape Town meeting.

Nigeria is the continent's largest economy, and South Africa is the second-largest. Both countries were already reluctant participants in the accord, which is supposed to help knock down the many barriers to trade among African countries.

Anti-immigrant sentiment is a long-standing issue in South Africa, where the legacies of colonialism and apartheid run deep, and a political shift has not delivered meaningful change to many poor South Africans. Immigrants from countries like Nigeria, Mozambique, Somalia and Zimbabwe are often regarded by South Africans as competitors for jobs and social services.

In South Africa, attacks on foreigners have become common, and they surged beginning on Sunday when rioters stormed neighbourhoods in and around Johannesburg, lighting fires and breaking into shops.

At least 10 people have died in the riots, President Cyril Ramaphosa said in a video address on Thursday, in which he also condemned the violence.

"There can be no excuse for the attacks on the homes and businesses of foreign nationals," he said. "Equally, there is no justification for the looting and destruction of businesses owned by South Africans."

In Gauteng, the province that includes Johannesburg, authorities have arrested at least 423 people, said Colonel Lungelo Dlamini, a police spokesman. On Thursday, he said that many shops owned by foreigners remained closed and that more shopping centres in the eastern part of the province "are being targeted."

Police seized guns, he said, not just from South Africans, but also from at least two foreign nationals.

The rolling backlash has united broad swaths of the continent. Two popular Nigerian musicians, Burna Boy and Tiwa Savage, said they were boycotting South Africa. Burna Boy was set to headline the Afropunk festival in Johannesburg in December, alongside artists like Solange Knowles. Tiwa Savage had an appearance in South Africa scheduled for mid-September.

On Tuesday and Wednesday, protesters rushed and sometimes looted South African-owned businesses in Nigeria and Zambia, including Shoprite supermarkets. The company closed stores. The South African telecommunications giant MTN did the same.

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On Thursday, the protests spread to the Democratic Republic of Congo, where demonstrators outside the South African Embassy in Kinshasa held signs that read "Don't kill our brothers" and "No xenophobia." In Lubumbashi, they broke windows at the South African Consulate.

Nigeria recalled its ambassador to South Africa. South Africa has shuttered its diplomatic missions in Nigeria, citing threats.

The clashes cast a cloud over the World Economic Forum in Africa, which began in Cape Town on Wednesday. Leaders were set to discuss the free trade pact, an agreement signed by 54 countries that supporters have said could reshape economic relationships on the continent.

The accord has the potential to bolster intra-African trade by 52 per cent by 2022, according to the United Nations. Right now, intra-African trade accounts for just 16 per cent of the continent's trade volume. It can be cheaper to ship something from Nigeria to Europe, and then to Senegal, rather than directly from Nigeria to Senegal. This is a major barrier to regional development, economists say.

Still, a host of challenges await before the pact is put in place.

African analysts differed on whether Nigeria's decision to skip the Cape Town meeting would have any impact in the long term.

Gilbert Khadiagala, a Kenyan professor of international relations at the University of Witwatersrand in Johannesburg, said Nigeria's move was little more than "grandstanding" and would not impede the trade agreement.

But Leye, of SBM Intelligence in Nigeria, said that in his view, Nigeria's boycott "will have an impact in terms of the pace of implementation."

The New York Times