This is the most controversial point in the interpretation of the movie, and the comments below show very different points of view. It contains the secret of the question, "Who is he?", and you will get very different answers depending on whether you take it for a return of the dead or an image for the film audience of the many people who make up V that is not 'real' in Evey's and Finch's reality - they could not walk out and meet Valerie etc. The plain reading is that they were dead before but alive and real at the end. They are those he promised to Vindicate in the V-monologue and the Vigilant, that is, those who wait at midnight. In this reading, the death of V has raised the dead; consistent with a good deal of messianic images and the Paschal tone of the final scene. If so, it's grounded in a gnostic or Arian Christology. The 'new world' is not shown, but is more than a political change. Alternatively, they aren't there physically and it's not a film blooper. They're there in spirit, carried in the hearts and minds of those who really do stand there. It's also notable that this is the only scene in the film (apart from the flashbacks) where any black people are seen. The graphic novel was more explicit about the fact that the Norsefire party had performed extensive racial cleansing in England, whereas the film merely hinted at it, focusing more on the persecution of homosexuals.
This is a physical representation of one of V's guiding principles: that you can't kill an idea; each of them represents part of the idea of freedom and self-expression. An idea, literally, is a form or archetype - so this is physically shown in the people's being antitypes of V. The obvious symbolism here, coupled with that present from the beginning of the movie with regards to a Guy Fawkes comparison and the constant mention of 'ideas' suggests that it was an intentionally symbolic scene. This 'idea' being the most important element of the movie, it makes sense that it would be steadily expanded upon and culminate in its most visceral representation during the movie's climactic final scene. V leads into it himself, during Creedy's death, by suggesting that he is still alive due to the fact that he is not just a man: "beneath this mask there is more than just flesh... beneath this mask, there is an idea," despite the fact that we have alternative and deliberately presented reasons as to why he is alive, if only briefly. With the explosion of the Parliament building, the 'idea' culminates in the visual depiction of various individuals who fought for this idea... people we know to already be dead. The people are still dead, but the idea is alive, the idea has triumphed. This scene can also be interpreted to illustrate what Evey said to Inspector Finch at the finale: "He (V) was Edmond Dantés (the hero of "The Count of Monte Cristo")... and he was my father. And my mother... my brother... my friend. He was you... and me. He was all of us, " meaning that V did it all for them (Gordon, the little girl with glasses, Evey's parents, etc.). In this sense, V symbolized the victims, both living and dead, of the government's atrocities, especially those who had questioned or fought against the oppression of the government.
The most literal explanation would be that none of them actually did die; we never see any of these characters die on-screen. Gordon is black-bagged, the girl is only shot once, and the two lesbians are killed/black-bagged in a possibly subjective flashback. However, this explanation stretches plausibility for a number of reasons: While we never see Gordon die, he is arrested and beaten, and it seems highly unlikely he would have simply been released, in light of his collection of forbidden items as well as his mockery of Sutler. We do see Valerie's body in a mass grave in an earlier flashback, one not based on the contents of her letter, and she and Ruth appear in the crowd without having aged at all in the twenty years since their arrest. Finally, we would also have to believe that the parents of a child suffering a gunshot wound to the chest would take her out of the hospital and involve her in a potentially violent conflict with armed soldiers. Moreover, judging by the view on the little girl's body right after she is shot (immobility, non-living eyes), it's very fair to assume she is dead. While the girl did not die (and was not shot) in the graphic novel, Ruth, Valerie, and Gordon definitely did.
This is a theme along the lines of, "an idea never dies." For instance, from Evey's first lines: "...We are told to remember the idea, not the man, because a man can fail. He can be caught, he can be killed and forgotten, but 400 years later, an idea can still change the world. I've witnessed first hand the power of ideas, I've seen people kill in the name of them, and die defending them... but you cannot kiss an idea, cannot touch it, or hold it... ideas do not bleed, they do not feel pain, they do not love... And it is not an idea that I miss, it is a man... A man that made me remember the Fifth of November. A man that I will never forget. " And then the scene with Creedy emptying his pistol trying to kill V : "Beneath this mask there is more than flesh. Beneath this mask there is an idea, Mr. Creedy, and ideas are bulletproof." By showing the people that have been slain, it shows that their idea, THE idea, lives on.