Employed and additional freelance work

No problem. Go ahead. If it same type of work and your work secretes etc. then take permission from the head of the department of your existing co. 

Usually the service conditions and service rules and regulations referred to in appointment letter also mention that employee can not be in gainful employment elsewhere.........................and in such case this may amount to dual employment/misconduct and warrant termination..........

If current employer agrees in writing then employee has a safeguard as per agreement. 

I agree  that it depends upon the terms of employment with the present employer, if it specifically mentions that the employee cannot undertake any gainful employment outside, then it is an obligation  on the part of the employee to not to undertake such offers.

I agree with expert Mr. umar Doab, it depends upon the type of service you are already in full time job? Does it put any sort of restrictions for any other (even part time) assignment/work?