***** NOT A TEACHER *****
I am 99.99% sure that the answer is NO.
"In what has turned into a ritual denial of the event" is, IMHO, a parenthetical remark "thrown" into the sentence by
the writer. Maybe some books might call it a prepositional phrase being used in an absolute way. That is, it has no
grammatical connection to the basic sentence.
The basic sentence is:
This Monday, Chinese authorities rounded up hundreds of activists in Beijing before they could gather to mark the occasion.
"This Monday" modifies the whole sentence, or it modifies the verb "rounded up." It depends on what book you wish to
Perhaps I can more clearly bring out these ideas if I wrote that sentence like this:
This Monday (in what has turned into a ritual denial of the event) Chinese authorities ....
This Monday, Chinese authorities -- in what has turned into a ritual denial of the event -- rounded up ....
HAVE A NICE DAY!
Student or Learner