Trade union activists say their annual May Day march will commemorate two of the titans of their movement; the former leader of the RMT Bob Crow and the former Labour MP Tony Benn.
The London May Day Organising Committee said it wanted to pay tribute to their work but also put across the message that the movement should not just mourn but also organise to fight for rights for workers.
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The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.
The family of the police officer who was murdered in the Westminster attack have thanked those who were with him 'in his last moments'.