A court has dashed the hopes of thousands of Palestinian supporters hoping to assemble at the Sydney Opera House, ruling their assembly unauthorised.

The verdict on Thursday, allowing police to move on or arrest those in the forecourt, comes after police challenged the Palestine Action Group's proposed protest in the NSW Court of Appeal.

Organisers believed Sunday's march would see around 40,000 people wind through Sydney's city centre to the steps of the famed landmark.

Justice Stephen Free said a protest of that size would have "given rise to a risk of crowd crush", a unanimous view of the court, he said.

Judges had raised concerns over crowd safety during earlier hearings.

Comparing the protest to a massive August rally at the Sydney Harbour Bridge, where between 90,000 and 300,000

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