Is online blackmail or extortion using private photos a criminal offence?

Yes, this is a serious criminal offence, covering both extortion (using threats to coerce you into paying money or complying with demands) and, given the private/intimate nature of the material, additional specific offences under the IT Act relating to violation of privacy and publishing or threatening to publish private images without consent.

It's important to act quickly but carefully — don't comply with the demands, since this rarely stops the harassment and can encourage further extortion attempts, and instead preserve all evidence (screenshots of messages, threats, any payment demands) without deleting anything.

Report this immediately to the cyber crime cell, either through the national cyber crime portal or in person at a police station — such complaints are handled with sensitivity and confidentiality given their nature, and prompt reporting significantly improves the chances of tracing the perpetrator and preventing further harm. It would also be worth consulting a lawyer who can guide you through the reporting process and any protective legal steps available in the meantime.