International supermarket giant Carrefour's shares plunged in Brazil on Monday, days after two white security guards at one of its stores killed a black customer, triggering widespread protests.

Shares in the French group's Brazilian unit were down six percent in afternoon trading on the Sao Paulo stock exchange after a video went viral of the guards beating 40-year-old welder Joao Alberto Silveira Freitas to death outside one of its stores in the southern city of Porto Alegre.

Carrefour shares suffered the biggest losses on the day, even as the Ibovespa index overall gained around one percent.

Products burn at a Carrefour supermarket in Sao Paulo during a protest against racism
Products burn at a Carrefour supermarket in Sao Paulo during a protest against racism AFP / Nelson ALMEIDA

In Paris, Carrefour shares closed down 2.21 percent.

Carrefour chief executive Alexandre Bompard was swift to condemn the killing as a "horrible act," send his condolences to the victim's family and cut ties with the private security company that supplied the two guards.

But Carrefour has nevertheless faced a wave of boycott calls and sometimes violent protests outside its stores across Brazil, drawing comparisons with the killing of George Floyd in the United States in May and the ensuing protests.

The killing came on the eve of Black Consciousness Day in Brazil, where around 55 percent of the population of 212 million identifies as black or mixed-race.