An unstable and emotional temporary social group with a leader is known as –

Crowd and mob are related terms. Crowd means a large number of persons gathered together. A crowd can be a crowd of random people doing different things or it may be people with one intent or purpose. Mob is highly emotional crowd whose members have with one intent or purpose and are engaged in or are ready to engage in, violence against specific target: a person, a category of people or physical property.