"Por favor, freie."
"Pelo amor de Deus" is the correct: "pelo" is the contraction of preposition "por" + masculin article "o" (logicaly it would be "poro" but that doesn't exist, it is "pelo" - or "pela" when the noun is feminin). Other examples: "Anda pela calçada" (Walk on the sidewalk). You can't say "Anda por a calçada" because the noun is defined. If you said "Anda por calçadas" you would be saying "Walk on sidewalks", like a general advice. It seems to me that, since the love is defined (it is God's love), you should use the article, therefore, the contraction "pelo" (instead of "por o amor de Deus" that is incorrect).
We don't usually say that usually (at least in the US) though. Maybe "hit the brakes" (which really means "stop", so maybe not what you want as a translation for this) or "slow down" (even "slow up" which isn't common but it's close to "hold up" would be understood quickly by most). It might be because "brake" sounds like "break". Usually if it's an emergency, you simply say "stop", but that would be "pare" or maybe "para" (I can't remember which) in Portuguese. Maybe they don't have to change it, but keep in mind, if you say that to someone you just met, they will pause a little trying to figure out what you are saying, while "slow down", "hit the brakes", and "stop" are almost immediately understood (in the US anyway).
Edit: Saying "whoa", "hold up", or screaming works if you want someone to slow down.
Thanks for a lot of emergency phrases, TerraZe :-)
As I responded above, "Por favor, freie" would translate to "Please slow down" whereas the verb 'parar', which you mention, translates to English "stop" (which is different). The verb 'to brake' is the former (i.e. doesn't imply a full stop), and even though I get your point, there's actually nothing wrong with the verb itself.
Jumping to imperative, the (positive/affirmative) imperative form of the verbs is generally the same as the present subjunctive, except for the 2nd person tu and vós, for which the imperative is formed by taking away the final 's' of the present indicative forms.
Or...to attempt to express it a bit more practical, you would (except for the 2nd persons again) generally change the last vowel from the form it has in 3rd person, present indicative (the "simple present" form) so that "a verbs" (the ones ending in -ar) will get an 'e' instead of the ending 'a', and the "i and e verbs" (ending in -ir/-er) will get a final 'a' instead.
For 'parar', the "simple present" is 'para' (as in 'Ele para aqui' = 'he stops here'). (For 2nd person, the "simple present" (i.e. indicative) would be 'tu paras'.)
Now, take the indicative present 'para' and switch the final 'a' into 'e', and you have the imperative form "Pare!" which is what you would shout. (For the 'tu' form, just take away the final 's' and make it "Para!")
- tu para ------- vós parai
- você pare ---- vocês parem
- (nós paremos)
So...as you can see, both your suggestion apply, depending on whether you use the 'tu' or 'você' forms ;-)