Regarding Joining a new vendor for client

Hello all, I am currently serving my notice period in a Indian MNC and working for an U.S client . Client is leaving this company and moving to a new vendor which is not a big mnc like my current company . I have received offer to join the new vendor and decided to join them . My current company has forced me to join a notice in a sheet a paper stating that I should not join client or third party of client for next 6 months and they may take legal actions if I join . I didn't have this non compete in my offer letter but signed it in a separate sheet just before month of relieving this company as the HR's started to threaten me and stop the exit process if I don't sign . Is this signed document valid ? Generally a non compete like this should be signed while joining but in my offer this is not mentioned in any means . Also from the new company they assured to take care of this if any issues arise . Please advice if I should stop joining new company , can my current employer take any legal actions with the notice signed now ?