Hundreds of Boca Juniors have fans gathered outside their team's hotel in Madrid on Saturday, chanting "It doesn't matter where you play, we will always follow you."
A few hours later, thousands of River fans packed the iconic Plaza del Sol in the heart of the Spanish capital, a day before the bitter Argentine rivals meet in the second leg of the final of the Copa Libertadores, South America's equivalent of the Champions League.
Tens of thousands of Boca and River fans are in the city for the "superclasico" on Sunday.
"I think fans from both teams are very nervous today," 14-year-old Boca fan Joaquim Naser, who travelled from Argentina to watch the game, said.
"It's going to be a great party at the Bernabeu. It's going to be packed with South American fans."
About 25,000 supporters from each club are expected at the game, with many making the trip of more than 10,000 kilometres from Argentina.
Both teams had closed practice sessions in the last day of preparations for the final, which was moved to Madrid after fan violence marred the second leg in Buenos Aires two weeks ago.
Boca Juniors players were injured before the match when River fans attacked the Boca team bus with rocks, bottles and wood. The first leg, hosted by Boca in Buenos Aires, ended in a 2-2 draw.
"Hopefully we will enjoy some of the atmosphere that we would have had with the match in Argentina," said Nacho Sorroche, a Boca fan who lives in Madrid.
Boca's latest attempt to be declared champions without playing on Sunday failed when the Court of Arbitration for Sport denied their request to suspend the second leg.
Boca appealed to CAS on Friday after South American soccer governing body CONMEBOL ruled against disqualifying River from the tournament.
Nearly 80,000 fans are expected at the Bernabeu, with supporters from each club placed at opposite sides of the stadium.
There will also be separate fan zones prepared for them near the Bernabeu with almost 4,000 security personnel deployed for Sunday's "high risk" match, nearly double the number used in the "clasicos" between Real Madrid and Barcelona.