What is this 60s or 70s espionage thriller?

Hi all. I think this is a 60s or 70s cold war spy thriller, but could be an international crime thriller. I remember that the location of the movie was possibly at a European Expo, and one scene is stuck in my mind: A man, a scientist I think, is the target of an assassination plot. They assassins decide to deliver a parcel bomb to him disguised as a present. The present/bomb is delivered, and the assassins realize, too late, that he has his family and grandchild/child with him and they end up killing them all. I think this takes place in a European city where an Expo is taking place. I thought it may have been Quiller, or the Naked Runner, but don't think so. Any help is gratefully appreciated.