Get yourself to the front of the crowd and face them.
To appear confident scan the crowd and ignore rudeness from individuals.
Speak loudly and calmly - use a few crisp clear words (words with consonants convey authority better).
Look at the people 3 rows back - it is they you need to address if you want them to move.
Use gestures to accompany your words (people on the edge of the crowd will not hear you and some may not be English speakers).