The Chicago convention in 1968 was marked by protests and violence.
Members of Chicago Copwatch rallied at Union Park and then went to the Fraternal Order of Police hall where the reunion was taking place.
Reunion organizers say the idea was to bring together old friends and recognize officers who were on the force at that time.
"It was a hard time for a new police officer on the force. I was young, the protesters were young," said William Jaconetti, retired Chicago police officer.
"They were attacking us not because of what we'd done, but who we were and what we believed in," said Don Rose,1968 protester.
Many of the people protesting were not born in 1968.