Brian Hodgson, leader of the Labour group on Oxfordshire County Council, joined former Cabinet minister Tony Benn and CND's Bruce Kent in speaking at the annual Levellers' Day event in Burford.
About 600 trade unionists, socialists, and peace campaigners joined the traditional parade on Saturday, before gathering at the church to lay wreathes and sing the Red Flag.
The event commemorated the execution in 1649 of three Leveller mutineers by Oliver Cromwell, following the English Civil War.
The men, who were refusing to fight a war in Ireland, were imprisoned and executed outside Burford Church.
Former Labour MP Mr Benn said: "This is an event that people come to year after year and which has repercussions way beyond Oxfordshire.
"The issues raised by those three Levellers are still relevant today.
"It is about fighting people that abuse their power, and saying 'no' to them."
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