Pandas transpose by columns and index

Question

I have a dataFrame as:

df=
     field_1       field_2
 10  123             456  
 20  789             101112
 30  131415          161718  

I want something like that:

index   field   value
 10    field_1    123
 10    field_2    456 
 20    field_1    789
 20    field_2    101112
 30    field_1    131415
 30    field_2    161718  

I tried to transpose the dataFrame but is not the same. I can't see any solution except interact row by row and save in a new dataFrame.