The Swedish activist addressed reporters in Athens, saying: "Our international systems are betraying Palestinians".
Ms Thunberg was deported from Israel after her detention on a Gaza-bound aid flotilla.
Delegations from Hamas and Israel have begun indirect talks to finalise a Gaza peace deal, with Donald Trump urging both sides to "move fast".
Read more: https://news.sky.com/story/gaza-latest-trump-tells-israel-stop-bombing-gaza-as-hamas-agrees-to-release-hostages-and-to-parts-of-peace-deal-13443931
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