Several thousand people joined the march, including other religious leaders. One group marched first from Hull, 250 miles away, wearing chains. William Wilberforce, the English abolitionist, represented Hull in parliament.
Last year, the Church of England, which owned slaves on plantations in the Caribbean, made a formal apology for past wrongs.
"The intention of today is not only to renew that act of repentance, not just an apology but repentance, acknowledgment that we were part of this terrible history, but also to wake people up to where we are now, the fact there still are problems," Williams said.
The 200th anniversary of abolition is Sunday. A national service is scheduled Tuesday at Westminster Abbey.