In Chapter Nine, when each of the contestants of the Hunger Games are interviewed, it is Peeta that is last, and it is Peeta that manages to steal the show by his confession of love for Katniss in front of all the cameras and Katniss herself. Note how the chapter ends with this revelation, that is a surprise to everyone, most of all Katniss. Having been asked why his love is so impossible, Peeta responds:
Peeta blushes beet red and stammers out. "Because... because... she came here with me."
This of course gains them both universal sympathy and support, as impossible love situations always manage to engage the public and the media. However, at the same time, as we consider the personal drama going on between Peeta and Katniss, this leaves Katniss very unsure of what is happening, as she doesn't know if this is just a cynical ploy of Peeta's to gain support for him or if this is genuine.
At the end of his interview, Caesar Flickerman, the host of the event, asks Peeta about his love life and whether there is a girl he likes. When Peeta answers yes, Caesar eagerly proposes that Peeta win the Hunger Games and then go back home the impressive hero. However, Peeta then reveals there is no way that scenario could ever occur because the girl he has a crush on, Katniss, is a fellow Tribute in the Games, meaning only one of them can survive the ordeal. This revelation shocks the audience, who is won over by the raw emotion and futility of their love (though Katniss has a very different take on the confession) and later allows the two to play up the "star-crossed lovers" angle while in the arena.
He confesses that he really loves Katniss. This was part of the plan to garner more donations from the districts but also an opportunity for Peeta to profess his love.
He reveals to people that he is in love with Katniss and Katniss was right there to hear what he had said