What "law" is Danforth referring to in his assertion: "I should hang ten thousand that dared to rise against the law, and an ocean of salt tears. ....

This line can be found in Act IV, Scene 1.  To find out
what law Danforth is referring to, just look a little bit farther back in this short
speech that he gives to Rev. Hale.  The law that Danforth is talking about here is God's
law.  Danforth is saying that he does not care at all how many people go against God's
law or how many people cry for him to be merciful -- he is going to uphold God's law as
he sees it.  This is why he will not pardon anyone or let anyone
off.

Here is the relevant
quote:

While I
speak God's law, I will not crack its voice with whimpering.  If retaliation is your
fear, know this -- that I should hang ten
thousand...