It is not just 'assistent', - this is 'assistent à' : "attend" or "to be present at", or "to witness" (60,000 spectators - 3rd person plural obviously).
The French is of course in present tense for the historical writing, while the English is translated to past tense, in keeping with typical use in both languages.
https://www.larousse.fr/dictionnaires/francais-anglais/assister/5829
https://www.lawlessfrench.com/grammar/historical-tenses/
PS: the use of 'assister à' in an interview reportedly created a lot of trouble for Gerard Depardieu, when it was translated in English news articles as 'participated (assisted) at', rather than 'witnessed', in reference to a vile crime. (article in Washington Post 1991)
