I think it is the communist salute, but if anyone knows better, please let us know. Have a quick look on Google images with "communist fist" as search words... I believe it is used more freely nowadays, as a sign of assent with anticapitalist/loosely Marxist/power to the people ideas. That's how I take it anyway, which is why I said I thought Dino Soldo was cool. It was perhaps a way for him of saying: "I'm on the side of the people, of the oppressed, Power to the People"! But of course, he alone can answer the question of what this clenched fist meant to him.
Not wishing to get into politics, especially as I am not qualified, I think that it is not quite fair to equate communism and Stalin. In France - where the particular person was disrupting the concert with her slogan - the far left scored highly in the 2007 general elections: the Revolutionary Communist League got 4%, the French Communist Party almost 2%, Workers' Struggle, Altermondialists and Workers' Party, 3% altogether. That's a total of 9% of all votes! I very much doubt that many of the voters were in favour of a communist dictatorship, purges, gulag labour camps... It was almost double that I believe in 2002 (17%), which explains partly why Le Pen made it to the second round, and people ended up having to choose between the right and the far right. So Leonard Cohen was certainly spot on when he claimed, as you quote, that "Notre époque connaît tellement de problèmes, c'est facile pour la droite de poser ses pièges."
Finally, "a communist freedom" can mean many things, probably something that has never existed, perhaps something that is utopian and can never exist, but it does not ring to me like Stalinism.
Hope this answers your post! I really don't wish to get into a defense of Communism (especially as I am not a supporter myself). As I said, I took the clenched fist salute to mean something cooler...
Edit: One last thought. You may want to remember that the Resistants that Leonard Cohen describes in The Partisan were almost all communists. As far as I know at least.