I have seen a few instances where the Indian police slap citizens as a fast punishment. Is that explicitly allowed by law, or just tolerated in practice?
The legal hook is reported to be §129 of the Code of Criminal Procedure, which authorizes use of force to disperse an illegal assembly, which this sort of is. No statute that I can find states that police can smack lawbreakers who are forced to disperse, but as is common in common law countries, the laws of India are not fully explicit on that which is allowed or forbidden for police to do. As this article indicates, systematic limits on police use of force remain to be developed.