Grosvenor Square 1968 has become a common piece of historical shorthand (Feature, Socialist Review, May 2008).
There was a demonstration against the Vietnam War in March 1968 in London. It did not go to the US embassy. The battle of Grosvenor Square, when the police mounted full scale cavalry charges, was in October 1967.
The March 1968 effort was much larger but much more passive. Tariq Ali led it off to Hyde Park, and only the ultra-lefts (yes, I was one) went to Grosvenor Square, where we were totally outnumbered by the police (who didn't lose it this time). Apparently, at the end, they all sang Auld Lang Syne together (no, I didn't). There was none of that in 1967.