Some 300 Guardia Civil police carried out 28 raids against the notorious Mara Salvatrucha, or MS-13, in their first such strike against the group, police said in a statement.
Law enforcement officers have reported a mounting threat from Latin American gangs in Spain, where economic hardship and high unemployment have left many young people feeling marginalized.
"MS-13 is a dangerous and violent criminal organization well represented in several American countries where they cause serious security problems and destabilization, especially in Central America, and which is trying to set itself up in Spain," the police statement said.
Officers detained 35 suspected leading gang members aged from 17 to 25 in the raids on Monday, some shown in police photographs bearing distinctive tatoos displaying their gang allegiance.
Police said they seized knives, fake guns, drugs and documents during raids in several Spanish regions, including the major cities of Madrid and Barcelona.
Other busts were carried out in the Mediterranean port of Alicante, where the stabbing of a South American at the end of 2012 sparked a long police investigation into MS-13 in Spain.
Madrid police chief inspector Ricardo Gabaldon said Latin American criminal organizations first appeared on the police radar in 2003, but have grown as immigration has increased.
Besides Spaniards, the suspected gang leaders arrested Monday included nationals of El Salvador, Romania, Ecuador, Bolivia, Honduras, Pakistan, Bulgaria and Morocco.
– 'School for delinquents' –
After first detecting the Latin Kings gang, born on the streets of New York and Chicago in the 1940s, investigators came across the Puerto Rican-origin Ñetas. In Spain, members of both gangs are mostly Ecuadorians, police say.
"Then came the Trinitarios, Dominicans Don't Play, and the Forty Two Gang," made up mostly of Dominicans but also Colombians, Bolivians and a few Spaniards, said Gabaldon.
He estimated there were some 300 street gang members in the Spanish capital, mostly aged 14-25 and based in the poorest districts of Madrid.
Part of a strict hierarchy with initiation rites, tests of courage, punishments and weekly quota payments, members are driven by inter-gang rivalry rather than violence against Spaniards or financial profit.
"The street gang is a form of apprenticeship, a school for delinquents," Gabaldon said. Later, some of their members make the jump to more serious organized crime.
"Their main goal is to show their bravery, to be respected. They are not looking so much to make money," he added.
In the tourist hotspot of the northeastern city of Barcelona, too, gang presence is growing.
Police there have recently raided the Latin Kings, Bloods and Black Panthers.
Gang activity in the port city is also becoming more dangerous, said the Catalan police director Manel Prat.
"These are groups that end up moving into drug trafficking, arms trafficking, extortion," he said.
Vladimir Paspuel, head of the Spain-Ecuador association Ruminahui, which seeks to help young gang members, said some members were driven by a sense of having being uprooted.
"They lay strong claim to their Latin American roots," Paspuel said.
"Many of them did not want to come to Spain at an age, adolescence, when it was hard for them to integrate into the groups that were already established at school," he added.
Their marginalization has been accentuated by Spain's economic crisis.
"In a country with 54-percent youth unemployment, what opportunity is society giving them?" asked Paspuel, who organizes anti-violence workshops for gang members.
"We are not offering them anything. We are offering them the street."