If you're simply experiencing poor service, fees, or delays, that's the proper complaint procedure.
Steps
However, if it's a lost card, a compromised OTP, a transfer or purchase you didn't authorize, or someone is trying to access your account, that's an urgent security alert: Secure your card or account and contact your bank immediately.
Once you've secured the situation, file a complaint about the transaction itself and obtain a reference number.
Don't wait for standard complaint procedures before addressing the risk; in fraud cases, preventing further losses takes priority.
The figures, fees and limits in this answer vary by bank and change over time — confirm them with your bank or the official source before acting.