A great number of hooligans were moving toward the stadium.
So the auxiliary 'to be' has to be adjusted to the subject. 'A number of smth' normally agrees with a plural verb (unlike 'the number of sth' which agrees with a singular verb).
Suggesting that 'moving toward the stadium' is a subject doesn't make any sense, though from the point of view of the formal sentence structure it may look like a subject.
- For Teachers