a religious group
The Mennonites were the first religious group to openly oppose slavery. Owning slaves was completely forbidden in this Christian community.
quakers
The Quakers strongly supported the abolition of slavery and founded the world's first anti-slavery society in 1775. They were a religious group known for their beliefs in social justice and equality, which led them to actively campaign against the institution of slavery.
quakers :} im pretty sure
The Quakers helped to lead the fight to end slavery. They were the first religious group to question the morality of slavery. They were instrumental in the successful operation of the 'Underground Railroad" that helped slaves escape to the northern state and then to Canada. You can read more about this interesting and egalitarian group at the related link.
The abolitionists were a group who fought against slavery.
Quakers were a religious group that played a significant role in the start of the Abolitionist Movement. Their beliefs in equality and nonviolence led them to actively campaign against slavery and advocate for its abolition in the United States.
The most prominent group of people that spoke out against slavery were the antebellum group that was termed anti slavery abolitionists. They were very outspoken and received allot of attention in the newspapers of the day.
The Quakers were the first group to publicly criticize slavery.
The first public stand against slavery in the Americas was in 1688 by the Quakers in Germantown, PA. They argued from the Golden Rule and from the principle against stealing, stealing other humans. The document was rediscovered twice, in 1844 and in 2005.
Many groups and individuals have protested against slavery throughout history. The first is likely to have been the slaves themselves. None have been successful in eradicating it.