you could give a general descrition of client, stating its leadership in industry and region.
for example, say, Lenovo,you can describe like: a leading PC and consumer electronic manufacturer in Asia. you may quota ranking from credible resource, i.e. gartner or CCID.
Nope, you are not supposed to list your client's name on resume. But you could do, for example, "Citi Group" => "Fortune 500 Financial service organization", "Time Warner" => "leading media company". Hope that helps.
I would talk to your manager and see which clients are public (clients they list in news stories) and list those. Specific names does add credibility and are easy to pick out.