SELMA, Ala. — Presidential candidates and prominent social justice activists descended on Alabama on Sunday to commemorate the anniversary of the brutal attack on civil rights marchers here in 1965, one of the most violent episodes in the struggle for black participation in democracy.

A who’s who of political figures, including five Democratic presidential candidates, were marking the occasion, nearly 55 years after the day that became known as “Bloody Sunday.” And Representative John Lewis of Georgia, 80, who announced in December that he had advanced pancreatic cancer, joined the march across the Edmund Pettus Bridge.

Mr. Lewis, who was 25 when his skull was fractured in the 1965 attack, told a crowd thronging him that they could help “redeem the soul of America.”

“We were beaten, we were tear-gassed. I thought I was going to die on this bridge. But somehow and some way, God almighty helped me here,” Mr. Lewis said. “We cannot give up now. We cannot give in. We must keep the faith, keep our eyes on the prize. We must go out and vote like we never, ever voted before.”