American Anti-slavery Society:

The American Anti-slavery society was one of the most well-known groups who opposed slavery. Started by William Lloyd Garrison in 1833 in Philadelphia, the society grew quickly and included over 250,000 members before 1840.
http://www.pbs.org/wnet/aaworld/reference/articles/american_antislavery_soc.html

Source: http://usinfo.org/docs/democracy/18.htm