Their proclamation comes on the back of Sunday's scintillating 4-0 drubbing of leaders Real Madrid at the Santiago Bernabeu.
A brace from Pierre-Emerick Aubameyang and a goal each from Ronald Araujo and Ferran Torres cut Madrid's lead at the top of the table to nine points.
Barcelona are in third place, 12 points behind Carlo Ancelotti's side but have a game in hand. There are still nine rounds of matches left to play.
"I don't know if we can win the title, we can't rule out anything," Xavi told reporters after the Clasico. "Maybe we're a bit late, but this is a very big win for us.
"We were much better than Real Madrid. We played almost as a home team in the the ground. It's a day to enjoy, not only as a Barca manager but also a fan.
"We could easily have scored five or six. We changed our negative dynamic of losing the last few Clasicos."
Busquets added: "If we still have a small chance to make a run for the title, we will fight until the end."
"It's very difficult but in football anything can happen. We are Barca."
Barcelona are unbeaten in their last 10 La Liga games, winning each of the last five.
