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The flag is a red star, in a yellow circle, in a green ring, on a red background.
The photo is from a protest under the Polish presidential palace against legitimizing the Turkish president Recip Erdogan, organized by a left wing, out-of-parliament "Razem"("Together") party.
Searching on flags.net didn't bring any results and I suspect it has to be an unofficial flag.
There are two more photos available in the link, but I can't link to them due to low reputation.
It's the flag of the PKK, the Kurdistan Workers Party:
They are in conflict with Turkey because of civil rights violations against Kurds, as well as a desire for a Kurdish nation.
This flag is the symbol of the Kurdistan Workers' Party, also known as the PKK. Their main goal is to establish an independent Kurdish state in a territory currently claimed by Turkey, Iraq and Syria.
Several states, including Turkey, The European Union and the United States, consider it a terrorist organization.
