What does Thoreau say is the only obligation he has the right to assume in "Civil Disobedience"?

In his letter from a Birmingham jail after he was arrested
during the Civil Rights movement, Martin Luther King takes his final words directly from
Thoreau:

I
submit that an individual who breaks a law that conscience tells him is unjust, and
willingly accepts the penalty by staying in jail to arouse the conscience of the
community over its injustice, is in reality expressing the very highest respect for
law.

In both cases, that of
Thoreau and that of King, they protest, not the rule of law, but the "tyranny of the
majority" as John Stuart Mill expressed the formation of rules called "laws" by
controlling groups. For, to use the term employed by Emerson, the "joint-stock company"
of society agree for their own interests that there should be a law. This law, be it one
for a war or one for segregation of races, is an unjust and should, therefore, be
challenged and rejected. Hence, Thoreau went to jail, and King also went to jail in
protest of the wrong of making something a law designed solely for the profit of others
that is, therefore, unjust. They both followed their consciences as every free man
should be allowed to do.