How were the underground railroad and free African Americans in the north affected by the fugitive slave law?

The Act compelled all citizens to report anyone who looked as thoughthey might be a runaway slave, on pain of a heavy fine. So any freed black was liable to be arrested on suspicion.

It was this Act which angered Harriet Beecher Stowe, and caused her to write 'Uncle Tom's Cabin' as her response. This best-selling novel drew attention to the Underground Railroad - the safe-house system that smuggled fugitive slaves into Canada - and recruited many people to the cause of Abolitionism.