In case your interested, the case is now solved.
I disconnected the battery and then connected the Magsafe unit: the MBP powered on and started the boot sequence. Disconnected the Magsafe, re-connected the battery and then re-connected the Magsafe: the MBP powered on again.
The battery indicator shows that the battery is completely discharged, but now it is charging again. I guess the incident did nothing good to it, but at least the MBP is up and running.