The red and black graffiti, with messages such as "4Innocent Muslims"; "Blair Lied Thousands Died"; and "J7 Truth", was cleaned off the columns of the memorial hours before families of those who died, survivors and emergency workers gathered for the ceremony in Hyde Park, central London.
Elizabeth Nicholson, a 31-year-old administrative worker from Bristol, whose sister, 24-year-old Jennifer, was killed in the Edgware Road bomb, said the event was a means for people to "show their love" for the victims and she had no interest in the apparent graffiti protest.
"I have no interest in dwelling on it (the vandalism) it is not something I consider to be important. I am here for my sister, I want to celebrate her," she said at the ceremony.
"She was very beautiful, vivacious, and full of life. She had a smile that if she walked into a room, it would light it up. She was such a beautiful human being. It feels like part of me has been ripped away.
"It is something that does not change, it is not a feeling that diminishes with time, it becomes part of your life."
Jacqui Putnam, who survived the bombing on the Edgware Road train, and has since become a Red Cross First Aider and a peace worker, said: "This is not a political memorial, this is not a political event, it is not there to make a political statement.
"What they did was a childish gesture and it has no place in an event like this."