Barcelona coach Pep Guardiola says his team's weekend defeat by Real Sociedad was a timely warning ahead of Tuesday's second leg of its Champions League semifinal with Real Madrid, which it leads 2-0.

Last week it came out on top in a stormy first leg in Madrid that was characterized more by cynical challenges and play-acting than on-pitch excitement.

Billed as a contest between the free-flowing soccer of Barca and the more direct approach of Real, it turned into a stop-start affair.

Pepe, deployed as a defensive midfielder by coach Jose Mourinho, was controversially dismissed after Barcelona full-back Dani Alves over-reacted to a challenge by the Portuguese international.

Barcelona's substitute goalkeeper Jose Pinto had earlier been given a red card for his part in a scuffle at halftime and Mourinho himself was sent to the stands after venting his fury at the referee.

It was an ugly encounter, with both coaches playing their part in the unsavory spectacle.

In the pre-match press conference Guardiola lost his cool, saying that Mourinho had won the battle of words but that he was ready to meet Real on the pitch.

After the match at the Bernabeu, an aggrieved Mourinho suggested European governing body UEFA favored Barca and even suggested it was due to its promotion of the humanitarian charity UNICEF.

The game's saving grace was the performance of Lionel Messi, who scored a double, the second being an excellent run where he slalomed through the heart of the Real defense before slotting past keeper Iker Casillas.

Barca is now in the driving seat to win a league and Champions League double, with an eight-point lead in La Liga after both it and Real surprisingly lost at the weekend.

"This was a lesson for us showing that nothing has been done yet," said Guardiola after the 2-1 defeat at Sociedad.

"Madrid are a very good team and they have nothing to lose so we have to keep our heads. We are going to have some tough moments in the game and we will need the support of our fans."

Barca went into its match in relaxed mood, with Real having already lost 3-2 at home to a Real Zaragoza side fighting for its top-flight survival.

Guardiola introduced several youngsters and it was 20-year-old Thiago Alcantara who gave it the lead but Sociedad pulled level and then Javier Mascherano conceded a late penalty, from which Sociedad scored the winner.

"It was a difficult match with a number of changes to the team and so I think we did relatively well, but in the second half we were not controlled enough in our play," said Guardiola.