After approximately 20 years of campaigning and political pressure, the Society for the Abolition of Slavery, or Abolitionist Movement, succeeded in passing the Abolition of the Slave Trade Act in Great Britain in 1807. In the United States, the movement swayed public opinion in the North in the years prior to the Civil War.
In 1806, a bill was passed in Parliament banning any British involvement in the slave trade with France. That had the effect of reducing the slave trade by two-thirds and set the stage for the 1807 Act which abolished slavery in all British colonies and made it illegal to carry slaves on British ships.
The law didn't require the actual freeing of slaves. The Abolitionist Movement was paralleled in the U.S., which, as an independent nation, did not adhere to the British law. By 1804, Abolitionists had pressured the northern states into a policy of gradually eliminating slaves.
The American Anti-Slavery Society, with the radical goal of eliminating slavery immediately, was formed in 1833. While there was public support for the movement, there was also opposition in both the North and South, whose wealth relied on slavery. There were sometimes violent opposition, including burning mailbags with Abolitionist propaganda. The Abolitionist movement began to fracture with internal dissent in the 1840s and became largely irrelevant, but had left a permanent mark on the society of the North.