PORTO ALEGRE, Brazil, Nov 23 (Reuters) - Supermarket chain Carrefour Brasil's share price plummeted 6% in afternoon trading on Monday after the death of a Black man beaten by security guards last week at one of its Brazil stores, igniting protests due to resume later in the day.

Shares in the Brazilian chain, the local unit of France's Carrefour, were the biggest losers on Brazil's Bovespa benchmark stock exchange index, which was up about 1% overall in afternoon trading.

A Carrefour Brasil spokesman declined to immediately comment on the fall in share price.

Some protesters have called for a boycott of the chain after a graphic video circulated on social media showing two security guards punching and pinning to the ground 40-year-old João Alberto Silveira Freitas on Thursday night. The two guards have been arrested.

The killing sparked Black Lives Matter protests in major Brazilian cities on Friday, with more than 1,000 demonstrators gathering at the store in the city of Porto Alegre where the incident took place. Some of the protesters rioted, destroying windows and vehicles, and setting fires.

Carrefour's global chairman and CEO Alexandre Bompard said late on Friday that the images of the beating were "unbearable" and he condemned racism and violence.

Carrefour Brasil said that it was taking steps to ensure those responsible were legally punished and that it would terminate its contract with the firm that provided security at the store.

Another protest was planned for Monday evening in Porto Alegre, organized by local Black activists, Black city council members and the third-tier São José soccer club, which Silveira Freitas supported. (Reporting by Diego Vara in Porto Alegre and Jake Spring in Brasilia; Editing by Howard Goller)