Proving that age is no boundary to publishing success, the French book world has been taken by storm by a surprise Christmas bestseller: a political call to arms by Stéphane Hessel, 93.

The unlikely publishing sensation is a former resistance hero whose 30-page essay, Indignez-vous!, calls on readers to get angry about the state of modern society.

Launched in October by Indigène, a small publisher working out of an attic in Montpellier, southern France, the book had a tiny first print-run, 6,000, and sold for €3, unprecedentedly cheap in a country where book prices are regulated and kept high by the law.

Hessel's success has stunned France. After two months on the bestseller lists, the book has spent five weeks at number one, beating Michel Houellebecq's award-winning latest novel La Carte et le Territoire and a host of Christmas fiction. It has sold 600,000 copies and – publishers predict it will reach a million. Translations are underway for Italy and other European markets.

The book's soaring sales reflect a general mood of French exasperation at the social inequalities of Nicolas Sarkozy's presidency. But the phenomenon is mostly down to Hessel's charisma and his life story.

Hessel was born in Berlin in 1917 and emigrated to France aged seven. His free-spirited mother, Helen Grund-Hessel, inspired the novel Jules et Jim, which became Francois Truffaut's film about a love-triangle of two male friends and a woman who loves them both. During the Nazi occupation of France, Hessel joined the French resistance, was caught, tortured and and deported to Buchenwald and Dora concentration camps where he escaped hanging. After the war, he helped to draft the universal declaration of human rights and later became a diplomat.

Hessel's book argues that French people should re-embrace the values of the French resistance, which have been lost, which was driven by indignation, and French people need to get outraged again. "This is an appeal to citizens, young and old, to take responsibility for the things in our society that don't work," he said. "I wish every one of you to find your own reason for indignation. It's precious." Hessel's reasons for personal outrage include the growing gap between the very rich and the very poor, France's shocking treatment of its illegal immigrants, the need to re-establish a free press, protecting the environment, the plight of Palestinians and the importance of protecting the French welfare system. He calls for peaceful and non-violent insurrection.

Sylvie Crossman, a former Le Monde foreign correspondent who co-founded Hessel's publishers, said the book was like a new, "adapted" version of Charles de Gaulle's rallying resistance appeal from London on 18 June 1940. She said the book had been a success because it gave hope to people from a real fighter who was not just an armchair intellectual.