The exact answer will depend on the documents you signed with your employer and the what could be the legal implications of those documents can be best explained by a legal consultant. I am not a legal consultant and what I am sharing is what numerous individuals are doing.
Code of Conduct
In MNCs, there is a code of conduct that every employee has to agree that includes single employment and many other details that you need to follow including the use of the internet, dress code…
Single employment is normally used for situations when you resign from your current job and will join the next employer only after release from your current employer and not before the release date from your current employment.
So if you have such a contract then you are not eligible for freelancing either.
There is an option that many freelancers use.
The person hired or paid and the freelancer who actually worked are different.
The freelancing client actually hires and pays in the name of your Wife / Husband / Mother but you get the job done for him. Normally clients don’t have issues with such a scenario as they are more interested in getting the job done.
Ethically if you are not entitled to be working, this workaround could be a legitimate way out but I am not sure. I see it being implemented by various freelancers.
Some MNC’s have such rules which they never enforce very strictly.
As an example, all companies have rules of no personal email in office but few minutes of email check in the morning or at lunchtime is not something that is taken very seriously by any manager in any MNC. It’s just that you cannot waste all your time in the office checking personal email or social media.
When I was in a job, social media was not so much popular and so cannot say what it would be the case but, then I don’t think it will be too different from emails.
So if you are freelancing on weekends and employer work is not being hampered I don’t see much of an issue either. Again don’t take this as the final words and consult the legal advisor.
Remember you should never use the time in office to dedicate it towards freelancing. You reach office and instead of doing your employer’s work, you start working on your client job, this is unacceptable.
What could be illegal is if you use the copyrighted code of your employer for your freelancing clients. This is illegal as you are not only violating the code of conduct but are infringing the copyright.