Disagreeing is always a sensitive issue. If you know your opposite number, it’s that little easier. If on the other hand, you don’t know him/her/them, I would suggest you soften the blow, by adding one of the 5 following phrases before.:
“I’m afraid / I’m sorry / With (all) due respect ” …..then add one of these:
“I totally/completely/wholly disagree.”
“I don’t agree at all.”
“I disagree entirely.”
“I can’t go along with you (on that)”
“I beg to differ“.
sensitive (sensible), to soften the blow (den Shock (ein wenig) abmilden), with (all) due respect (Bei aller gebotenen Achtung)