I have just finished my MSc after studying full-time for the past year. Due to financial reasons I worked as well throughout the whole year except for 6 weeks in May-June when I took time off to study for my four final exams (they had scheduled them all to be sat within a 24hour period in mid-June). From October last year until end of April I worked 24.5hrs a week spread out over five days (two days 9am-5pm, three days 9am-12.30pm) and then from July until last week when I handed in my dissertation I worked full-time (35hrs/week). So yeah, it is possible to do it, but it's not recommended and it all really depends on the specific course that you're doing.
I do think it's a good idea to work part-time (paid work, volunteering, internship) if you can, just because it helps you keep one foot in the 'real' world and you are building up your CV with work experience as well as more education. Employers also really like to see that you are able to multi-task, stay organised enough to balance work and study etc. If I had aimed to get a Distinction on all assignments this would just not have worked, but I have been still able to keep a Merit average which is fine by me.
When you work and study at the same time you have to accept that you will have to give something else up. For me that has been my spare time, having a social life and having any kind of 'life' outside of working and studying over the past year. But I preferred to do my Masters over one year to get it done and over with more quickly. At least this way I had 'no life' for just the one year whereas if I had been working full-time and studying part-time for this degree I would have been this busy for two years. I strongly doubt that I could have kept my motivation levels up for that long.