Identify the leader (there is always a leader) and attack him rapidly, brutally and remorselesslly. The others will usually loose their nerve.
Good Advice! You want each of his buddies to say "I don't want that happenning to me!" The best method would be the creation of multiple sucking chest wounds in the main attacker but if you do not have a gun then use what you got.
Musketeers #1 Rule of Unarmed Combat: One only fights barehanded when naked and all the furniture is bolted to the floor. There is almost always something harder than your hand around to hit someone with. Use it and as another poster stated, go beyond the point of reason. Leave the main attacker a pasty smear. There are no rules when it is 2+ on 1.
Accept that you are going to get hurt, possibly killed. Focus on doign as much damage as possible and then use the opportuntiy to get away. Bite, Kick, Tear, Eye Gouge, your only hope is to make the drawbacks of attacking you enough to dissaude others.