Ramón Calderón spoke at a charity event ahead of the first Clasico without Cristiano Ronaldo and explained why he believes Madrid should not have sold him. (2:25)

La Liga's Clasico is the most watched domestic club game on the planet and nobody at the Camp Nou on Sunday will be more alone, in the unforgiving spotlight, than the referee.

Clasico officials must be prepared to take monumental flak before, during and after the 90 minutes, receiving it from players, coaches, fans and pundits leaning towards either Real Madrid and Barcelona, and while trying to make split-second decisions that potentially have long-lasting consequences.

Few referees know more about this atmosphere than former top La Liga ref Eduardo Iturralde Gonzalez, who both enjoyed and suffered in Clasicos during his 17 years in charge of La Liga games.

"You always have nerves before such a high-profile game," Iturralde says in an exclusive interview with ESPN FC. "All that happens before, you cannot control, people stopping you on the street, all the calls you keep getting in the weeks before the game. So you want the game to start, that is where you are in control.

"Once the ball is moving your stomach calms down and you are where you like to be, out on the pitch. Referees always like to whistle these type of games. But it's curious: it is a game that can take more away than it gives you. There are many very good referees who have been very good in Clasicos and it has not helped their careers."

A first big call was not long in coming in Iturralde's debut Clasico in February 1999 at the Camp Nou. The game was goal-less with 20 minutes gone when Madrid left-back Roberto Carlos chopped down his future teammate, Luis Figo. Iturralde immediately reached for his red card and Barca went on to win 3-0 against the 10 men of Madrid.

"I always say that a referee cannot think, he has to act," he explains. "As when he starts to think, he is not refereeing any more. If you see something, you whistle. If you start to think about whether to send this player off or not, he is from Madrid or Barcelona... you cannot do that."

Iturralde refereed three clasicos in his career. 'You always have nerves before such a high-profile game. So you want the game to start. That is where you are in control.' Bagu Blanco/Getty Images

Iturralde was often in the headlines as he called games in La Liga's top flight from 1995 until 2012, showing more yellow cards (1,647) and awarding more penalties (104) than any other referee in Spanish football history. But the Metallica fan, who trained in dentistry, was generally popular with players and and says that a referee cannot be star-struck by officiating the likes of Sergio Ramos or Lionel Messi.

"They are the same as a player from Almeria or Getafe, just much more high-profile," he says. "A referee cannot let themselves be influenced by these things. Even more now that referees in Spain are earning a lot of money: €300,000 a year. In a Clasico, a player knows that hundreds of millions of people are watching him. And he has his pride, he knows this is the game when he really has to do well. So they do protest. But when the game ends, or if you see them in the airport, they treat you like another fellow professional, a comrade."

To illustrate how players and officials can share a moment on the pitch, Iturralde recalls November 2010's famous victory by Pep Guardiola's Barcelona over Jose Mourinho's Madrid.

"The Camp Nou was falling down as it was almost over, and the game was at 5-0," he says. "I remember it perfectly. [Lionel] Messi picked up the ball near the touchline, started to dribble and [Sergio] Ramos came from behind and hacked him down. I sent him off, and a terrible push and shove broke out. And [Andres] Iniesta just came over to me and said "Itu, relax, enjoy it.' I couldn't believe he was so calm, with all that was going on."

The relationship with coaches can be less friendly, with Iturralde admitting he reacted when current Manchester United manager Mourinho tried some mind games ahead of that meeting.

"The day of the game Marca published a conversation with Mourinho saying that 'Iturralde is very good for Barca,'" he says. "That was to try and make me give doubtful decisions in favour of Madrid? Referees are well used to that by now. They know very well where the messages are coming from and why. You just have to be very sure of what you do and the decisions you make.

"If that annoys someone, well, bad luck for them, but no problem. So when the game was over I told him 'Looks like you got it wrong, in this there is only one big man here, and that's me.'"

Iturralde clearly enjoyed that evening at the Camp Nou, but it was to be the third and final Clasico of his career (the second was Madrid 0-3 Barca at the Bernabeu in November 2005). The Spanish authorities wanted him to take charge of the 2010-11 Copa del Rey final but the Bernabeu hierarchy vetoed his appointment.

"The committee said 'we want Iturralde,'" he says. "But Madrid did not want me, and an alternative was chosen by consensus. But that is normal, not something that bothers me. I understand that [Madrid] had lost all their Clasicos with me in charge, so they wanted somebody else. It's only in such games, like the Copa final or the World Cup, when politics enters."

Iturralde sent Ramos off in a 5-0 Barcelona win in 2010. 'I sent him off, and a terrible push and shove broke out. [Andres] Iniesta just came over to me and said 'relax, enjoy it.'' David Ramos/Getty Images

Sunday's referee is Jose Maria Sanchez Martinez, just 35, who last season took charge of two Clasicos, Madrid's 2-0 stroll in August 2017's Supercopa second leg and Barca's 3-0 victory at Bernabeu last December.

"He has done well, and everyone is happy," Iturralde says. "The more Clasicos he whistles, the more incidents will be looked at. With the veteran referees, people always talk about a decision from five or 10 years ago. If [Sanchez Martinez] does three or four more Clasicos, he will make a mistake -- everyone does -- and it will be just the same with him. That is how things work in Spain."

Making things more interesting is the fact that Sunday's will be the first La Liga Clasico with the VAR (video assistant referee) system, which Iturralde says helps the officials.

"VAR takes away that tension, when you wonder during a game if you have got a decision correct," he says. "Now you know you have got it right, if they have not told you. At this World Cup you saw it - as previously there were referees who could not even talk after a game, as they were still thinking they might have made a mistake, and would be sent home. Not any more, with VAR. Now you know you have this support."

Iturralde says that a first Clasico in a decade without either Cristiano Ronaldo or Messi, and with relatively less combustible characters Ernesto Valverde and Julen Lopetegui on the bench, will not necessarily be easier to control.

"It will be just the same," he says. "The game comes early in the season, week 10 of the season, but Madrid are not playing well, and if they lose... that means extra pressure. I believe it will be a physical game and the referee will whistle lots of fouls early. Then it depends on if the players relax and concentrate on playing.

"In the Clasicos, when the game is level, there is not too many problems for the referee. It is curious."

No pressure, then, for Sanchez Martinez on Sunday: only the whole world will be watching, with his future career prospects on the line.