Leeds United Supporters Trust and the Lions Trust have issued a joint statement following reports only 1,000 tickets would be made available for travelling fans in each leg.
Millwall lost at Carlisle on Saturday to end up in fifth place while Leeds beat Northampton to finish fourth and the two clubs will now meet in the first leg at the New Den on Saturday, May 9 and at Elland Road in the return on Thursday, May 14.
The statement, called 'Fans speak as one', read: "The Lions Trust, in tandem with the Leeds United Supporters Trust, are calling on the managements of the clubs and police commanders involved to work with fans to make both matches the fantastic occasions they deserve to be.
"The difficulties of policing important football matches and the security problems that they impose is of course recognised and acknowledged. We have every confidence, however, that the Metropolitan and the West Yorkshire police forces have both the professionalism and experience to be able to handle what remain, after all, third-tier level football matches."
Both fans' groups argue restrictions could force visiting fans to obtain tickets in the home sections, causing potentially more problems.