Dear Sameer,
Involvement in physical fight is far serious matter. It needs to be investigated by conducting domestic enquiry and punish the guilty with severe punishment. Warning letter is far mild punishment.
Handling the case from legal side apart, you need to check with the organisational climate also. Why situation came to such a pass? Why employees are so frustrated? Do they have sufficient resources to carry out their job? Have you demarcated the areas of responsibility? Do you have grievance handling policy and if yes then how do you check its effectiveness? What is the role of the manager? Is he goody-goody in front of his/her boss and behaves differently with the subordinates?
You may dismiss this incident as aberration but there appear to be far more deeper issues and you and your management need attention.
Thanks,
Dinesh V Divekar