The Tsunami Democratic protest group have tasked its supporters to meet close to Barcelona’s stadium four hours before kickoff.
Tensions have been high in Barcelona since nine Catalonia leaders were jailed in October for masterminding an alleged illegal referendum in 2017.
Originally, El Classico had been set for October 26, but civil unrest in Barcelona compelled La Liga authorities to reschedule the game for December 18.
In a statement posted on Twitter, the group said: "At this time, an important part of the Catalan population suffers from exclusion for ideological reasons.
"It cannot exercise fundamental rights without repression, nor can it exercise his right to self-determination. In addition, dozens of people are or have been arrested simply for exercising and promoting these rights.
"After verifying the impossibility of agreeing with FC Barcelona and Real Madrid CF the opportunity to make visible, in the stands and on the pitch, in an effective form, the call for dialogue that Spain ignores, Tsunami Democratic commits to doing so.
"For this, Tsunami Democratic convenes a massive concentration on December 18 at 4pm outside Camp Nou. So far, more than 18,000 people have signed up to the call.
"Many of these people, with tickets or membership [of FC Barcelona], have already received the relevant slogans and have organised to develop the actions planned for a day that will be followed by 650 million people around the world."
Real Madrid and Barcelona are both locked on 34 points from 15 games with the Spanish champions only topping the table on goal difference.
