Sentence:
He reached the city where the people were, the city which was on fire.
Should I use a comma, a semicolon, or a conjunction to connect the two clauses?
You are using a gapping comma to avoid repeating part of the first sentence fragment in the second.
He reached the city where the people were, (he reached) the city which was on fire
A semicolon would work:
He reached the city where the people were; the city which was on fire
but the two sentence fragments aren't independent, so I would consider its use inappropriate here.
A conjunction such as 'and' suggests that 'which' is dropped, and this changes the dramatic effect of the sentence:
He reached the city where the people were, and the city was on fire.
I'd stick with the comma.