BARCELONA, Spain — There are no membership fees, no roll calls, no official leaders and no headquarters. Attending the weekly meetings is optional.

They are loosely organized groups of neighborhood activists known as defense committees, and they have taken a key role in Catalonia’s independence movement after its political leadership was jailed or fled the country.

Ever since Spain’s crackdown on an unauthorized referendum on secession in October, hundreds of self-proclaimed Committees for the Defense of the Republic are waging a campaign of economic disruption, blocking roads and highways and temporarily seizing toll booths in defiance of the Spanish government.

Though the activists say they are non-violent, Spain sees their activities in a different light. A female activist was arrested this week on suspicion of terrorism while six other people were detained for public disturbances in connection with pro-independence protests in January.

The defense committees include people of all ages and walks of life. Josep, a 56-year-old economist who declined to give his last name for fear of being arrested, attended an assembly by the defense committee in his Barcelona neighborhood for the first time last week. He said he already belonged to two civil society groups campaigning for Catalan independence, but wanted to do more.

“Only good intentions, non-violent marches and yellow ribbons haven’t been enough,” he said, referring to the ribbons that many pro-independence activists wear in support of jailed Catalan leaders they regard as political prisoners.

The defense committees were created last year in around 60 towns across Catalonia. Originally named Committees for the Defense of the Referendum, their primary goal was to safeguard voting centers where the banned independence referendum was held on Oct. 1.

Today, members say there is a network of over 300 committees, also known as CDRs. Many have their own unique logos and separate profile pages on social media platforms, which they use to gather people to their protests.

Some rural CDRs have just four people, while others in Barcelona draw over 200 participants to their assemblies.

Protests also vary in style and size.

Last week in the town of Figueres, a protest organized by the local defense committee looked like a scene straight out of a horror movie. Dozens of participants wearing white masks stood in absolute silence for a half-hour at the local town square to call for the freedom of jailed separatists.

On Saturday in Barcelona other groups organized a 24-hour vigil in which over 200 people took turns walking in circles around a former penitentiary.

But amid heightened tensions in Catalonia, some defense committees engage in more impactful actions, like blocking highways and train stations.

“If you don’t disturb the economy, no one will listen to you,” said a 55-year-old female member of the CDR in Barcelona’s Eixample neighborhood. She declined to give her name because she feared arrest.

The woman arrested Tuesday was initially investigated for rebellion and terrorism but Spain’s National Court lowered the potential charges to public disturbances and released her, but barred her from leaving the country. She is suspected of being one of the network’s leaders, which the activists deny, saying they have no leaders.

Spanish newspaper El Pais published what it says is an audio message by the woman in which she describes plans for a major workers’ strike that never happened. The voice in the recording speaks vaguely of plans to paralyze Barcelona’s port, disrupt communication lines and block train tracks, “but without violence.”

The Spanish government views such actions as sabotage against the state. Interior minister Juan Ignacio Zoido has described the CDRs as “organized cells capable of causing damage, disturbances and of breaking normality.”

Still, some security experts say such violence should not be equated with terrorism.

“We could debate to a certain point if these actions can be considered violence. Not physical violence, but some form of violence,” said Sonia Andolz a political scientist and lecturer on international conflicts and security at the University of Barcelona. But for it to be considered terrorism, it would mean “having a political organization that wants to cause terror in the civil population,” she added, recalling Spain’s history of deadly attacks by Basque separatists and recently by Islamic extremists.

In a separate investigation, six men believed to be members of the CDRs were arrested Tuesday by Catalan police, which are under direct control by Madrid under emergency rules imposed after separatist leaders defiantly proclaimed Catalonia independent last fall. The men were released after questioning and face public disorder offenses for their participation in tense protests outside the Catalan parliament in January.

Hundreds of people protested against the arrests in Barcelona on Tuesday, holding up signs saying “I am CDR.” Mariano Alvarez, a 61-year-old Madrid native who has lived in Barcelona since 1984, said if Spanish authorities are trying to intimidate people like him from protesting in the streets, it may have the opposite effect.

“They cannot detain all of us,” he said.