Demonstrators brandishing Molotov cocktails, fireworks and clubs have taken to the streets of Athens.
Security forces are holding them back in some areas of the city with water cannon and shields.
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A police raid uncovered what is believed to be cocaine that had been stashed in nappies, and 40 cannabis plants.
The prince attended the rugby match in his role as patron of the Invictus Games Foundation, the official charity for this year's game.
The prime minister pitched her comments firmly towards unionists in her first campaign visit to Scotland ahead of the general election.