2003 MG ZR Car alarm won't stop

My car alarm won't stop going off. I disconnected the battery and the alarm was still going until I disconnected the wire going from the battery to the fuse box. I left it disconnected for about half an hour then reconnected it, the alarm instantly started again, I tried starting the car and it started for around 10 seconds then cut off again, while the car was started the alarm was still going off. Is this a battery problem or something else?

I have an MG ZR