Barcelona and Real Madrid face a new threat of disruption at the season's first Clasico after pro-Catalan independence activists targeted the Camp Nou for protests.
The Tsunami Democratic protest group, which is run by anonymous leaders and mobilises followers by means including social media, has called on its supporters to head to four meeting points close to Barcelona's stadium at 4pm local time on December 18.
That is four hours before kick-off in the biggest match of the LaLiga season so far, which was moved from October 26 to its December date following civil unrest in Catalonia.
Tsunami Democratic says more than 18,000 supporters have already committed to joining its protest, including many who have tickets to the Clasico and members - known as 'socios' - of home club Barcelona.
The jailing of nine Catalan separatist leaders sparked major protests in Barcelona during early autumn, and the Tsunami Democratic group was behind a move to bring the city's El Prat airport to a standstill by flooding it with protesters.
Now the same group, knowing it will be unable to protest inside Camp Nou, is intending to make itself impossible to avoid on the day of the match, which could mean political protests take place inside the stadium as millions watch across the world.
Tsunami Democratic said: "In an exceptional situation, it is necessary to act exceptionally."
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."