Barcelona remained eight points clear of Real Madrid in La Liga as the two rivals drew in the first of four games they will play in just over two weeks.
A Lionel Messi penalty gave Barca an early second-half lead after Raul Albiol hauled David Villa to the ground with a challenge that saw him sent off.
Real’s equaliser was also provided by a penalty when Daniel Alves brought down fellow Brazilian Marcelo.
Cristiano Ronaldo, who had earlier hit the post, fired home the spot-kick.
The two clubs now meet in the Copa del Rey final on 20 April, which will be followed by their Champions League semi-final.
Saturday’s El Clasico never quite morphed into a classic and at times was spoiled by some cynical challenges and gamesmanship, with even Messi reduced to kicking the ball into the stands at one point.
Real’s initial policy was one of containment – Mesut Ozil began the game on the bench – which was, perhaps, not such a surprising tactical decision given Mourinho’s team had been embarrassingly beaten by Barcelona 5-0 at the Nou Camp in November.
That strategy was to crowd the midfield in the hope of stifling Barca’s passing game, relying on Ronaldo and Angel di Maria to provide the home side’s counter-attacking threat.
It was a strategy that enabled Barcelona to dominate possession, though Real probably had the clearer openings in the first half.
Just before the half-hour Di Maria glimpsed goal only to miscue with a wild shot and soon after Ronaldo swept into the Barca area before Adriano raced across to take the ball away from the Portuguese winger.
Adriano’s fellow defenders – Carles Puyol and Gerard Pique – also made crucial interceptions in a niggly opening half to cut out passes from Ronaldo and Di Maria.
Just before the interval Ronaldo’s powerful header was headed off the line by Adriano following a Real corner.
It was Di Maria’s fellow Argentine Messi who carried the main Barca threat to Real in those opening 45 minutes.
Early on Messi attempted to lob Casillas and after a slaloming run by the Argentine international the Real Madrid goalkeeper did well to beat away the Barca forward’s shot.
Casillas also appeared to bring down Villa but referee Cesar Muniz Fernandez was unmoved apart from waving a yellow card at Pique for his protestations over the decision.
Early in the second half Ronaldo’s dipping free-kick clipped the upright but minutes later Real found themselves a man and a goal down.
If Villa’s first-half penalty appeal had been ignored by Fernandez the referee immediately pointed to the spot after Albiol crudely wrestled the former Valencia striker to the ground.
Messi’s penalty was not the best but Casillas dived early as the Argentine notched up his 49th goal of the season.
Barca then had to rejig their defence when Puyol, who was playing for the first time in three months following a knee injury, had to be substituted with what looked like a hamstring injury.
Not that the loss of their captain seemed to have any effect on Barca’s increasing dominance as Xavi Hernandez clipped the bar with a curling shot.
Mourinho introduced Ozil and the on-loan Emmanuel Adebayor as Real attempted to salvage points and pride from the game.
And Real managed to do just that when Marcelo went to ground after he was tackled by Alves and Ronaldo’s precise penalty provided the Portuguese’s forward with his 29th league goal of the season.
After Saturday’s league match the two clubs will meet in the Copa del Rey final on 20 April, which will be followed by their Champions League semi-final – with the first leg at Real’s Bernabeu stadium on 27 April and Barca staging the second at the Nou Camp on 3 May.
With six games left in La Liga, Real are likely to need a dramatic collapse by Barca for Mourinho’s side to win the league, though they can take credit from their ability to take a point from a game in which they played nearly half of it with 10 men.
“Eleven against 10 and it was practically mission impossible,” said Mourinho. “Especially against a team that – with possession of the ball – are the best in the world.
“We drew and nearly won but, with the circumstances of the game, it’s a draw you have to be happy with.”
Mourinho’s Barca counterpart Pep Guardiola said: “We should have attacked Casillas’ goal more when it was 1-0 but its natural to sit back some when you’re in the lead.”