A town in the south of France is holding a special church service to honour the heroic police officer and three other people killed in a gun rampage by an Islamic extremist.

The French police officer who swapped himself for a female supermarket employee being held hostage had already received a lifetime of accolades by the time he walked unarmed into the store under attack by an extremist gunman.

Latest: An armed man took hostages in a supermarket in southern France, killing three people and injuring about a dozen others before being shot dead when French police stormed the market, authorities have said.