This would appear to have been the first streak at a major sporting event anywhere in the world. It happened during the half–time interval of a rugby union match at Twickenham, between England and France.
There's a famous photograph of a police constable (PC Bruce Perry) covering O'Brien's private parts with his helmet, while a man in a suit runs towards them in the background with a raincoat at the ready. You can see the photo here.
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