Hayden died in his home in Santa Monica "after a lengthy illness", the Los Angeles Times reports.
He was a member of the "Chicago seven" charged with conspiracy over anti-Vietnam war protests in 1968 and eventually acquitted.
Hayden later served in the California state assembly and Senate for nearly two decades. He was married to actress Jane Fonda between 1973 and 1990.
Tom Hayden, famous anti-Vietnam war activist, dies aged 76
24 October 2016
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Famous American anti-war activist Tom Hayden has died aged 76.
