Complex sentence: His activism culminated when he was one of the leaders of a short-lived protest in 1848 during which inhabitants of Batavia (across ethnic divisions) presented their grievances to the local government.
Split into simpler sentences: His activism culminated when he acted as one of the leaders of a short-lived protest in 1848. During the event, a multi-ethnic group of Batavian inhabitants presented their grievances to the local government.