Guardiola congratulates 'champions' Madrid
The defeat left Madrid seven points clear with just four games left and comfortably within reach of their first La Liga title since 2008 and their 32nd in history.
"I congratulate Real Madrid for their victory and also for their league title which I have no doubt they have clinched tonight," said Guardiola, whose side had won the last three league titles.
"We played a good match, but it wasn't good enough. We played the kind of game that we needed to play but we just couldn't do it."
Guardiola also defended his decision to start the game with Gerard Pique and Cesc Fabregas on the bench.
"I selected the team which I thought had the best chance to defeat Madrid and, anyway, the losers are always wrong," he said. "Now we have to look to the future."
That was a reference to Tuesday's Champions League semi-final against Chelsea when the Spanish giants will attempt to overturn a 1-0 deficit from the first game at Stamford Bridge last week.
"Tonight, the players are sad, they are tired, but in their careers they have all faced similar situations and overcome them."
Real Madrid assistant coach Aitor Karanka, whose side lost 5-1 in the same fixture last season, refused to declare the title race was finished.
"This match was not decisive in the league title race," said Karanka, who again stood in at the post-match news conference for coach Jose Mourinho.
"There are still games and points to play for. Mourinho and the players are all happy, but we are also thinking now of the Champions League."
Real have a 2-1 semi-final deficit to overturn on Wednesday against Bayern Munich at their Santiago Bernabeu home.
"At half-time Mourinho spoke of the possibility that Barca could grab a goal and that it was crucial for us to chase a second goal. That is what happened."
Cristiano Ronaldo grabbed the winner in the 73rd minute on Saturday after Barca substitute Alexis Sanchez had hit a 70th-minute equaliser to Sami Khedira's first-half goal.
The goal was Ronaldo's 54th in all competitions this season.
"Ronaldo has given us lots of fantastic moments. For me, he is the best player in the world," said Karanka.
Ronaldo outshone fellow superstar Lionel Messi, who was largely anonymous, and later tweeted: "Mission accomplished".
On a memorable night for Madrid, their first goal was the 108th of their league campaign which beat the record previously held by the Real Madrid side of the 1989-90 season.
Madrid midfielder Xabi Alonso also refused to believe that the title race was over.
"It's a very important result, but it hasn't decided anything," said Alonso.
"It is a big step towards the title. We need to finish it off. We controlled the match tonight and the result was fantastic.
"The key moment was Ronaldo's goal, it was the quick response to their equaliser."
Barcelona's Sergio Busquets admitted the game was almost up for his team.
"We are not in our best form at the moment, but we stayed true to our style of play," he said.
"We will keep battling but it'll be difficult."
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