Original:  Beginning at 1pm CEST (1100 UTC), students, pupils and ordinary citizens marched through much of downtown Gieen in the biggest demostration the city has seen in years.
Revised:  Beginning at 1pm CEST (1100 UTC), students, pupils and ordinary citizens marched through much of downtown Gieen , accompanied by a massive police presence, in the largest demonstration the biggest demostration the city has seen in years.
Edit intent: fluency