Kate Middleton, Prince William, and Prince Harry all attended the Service of Hope at the London landmark where William and Kate tied the knot in 2011.
For the solemn occasion, the Duchess of Cambridge wore a black and white printed coat and matching hat, and her husband and brother-in-law sported matching black suits.
Ahead of the service, Prince William laid a wreath at the Innocent Victims Memorial outside of the Abbey a short distance from where the attacks took place.
On March 22, a driver plowed through people on the popular Westminster Bridge, killing three, before fatally stabbing a police officer outside the Houses of Parliament. The terrorist was then shot and killed by police.