A lone woman armed with a whistle and enormous courage has set herself the task of defending unsuspecting tourists from the pickpockets who plague Barcelona.

Eliana Guerrero became so incensed by the way groups of thieves were able to freely roam the city's underground network that she now patrols the metro on her own.

She gives loud blasts on her whistle whenever she sees the thieves, who operate mostly in a dozen city centre stations including the Sagrada Familia and Placa Catalunya, and hands out flyers warning about the criminals.

"They normally run off when they hear the whistle," Guerrero told the Guardian. "But sometimes they threaten me and last week, after I stopped them robbing someone, one of them grabbed me and sprained my finger."

Guerrero, 38, started her campaign after seeing a thief snatch a bag from a German tourist, look inside and then hurl it away – smashing the insulin containers inside, despite the pleas of the tourist involved.

"I couldn't believe that they would do that. Why would they throw away a diabetic's insulin?" she asked.

She began by simply trailing the pickpockets and shouting out warnings while handing out the flyers that she prints herself, but found that not enough people could hear her – which is why she bought a whistle.

"The whistle is what really gets them," she says. "That, and a sign I hold up that tells people, in four different languages, there are pickpockets at work."

About 150 bag-snatchers operate on Barcelona's metro, trains and buses, committing 90 robberies a day, according to police. The thieves form well-organised gangs that often co-ordinate the shifts they work.

Guerrero blames Spanish law for the impunity enjoyed by Barcelona's pickpockets.

"If they take less than €400 [£350], then they only get a small fine – and then only if the victim turns up to give evidence, which is why they target tourists," she said. "And the police don't have a database that enables them to tell if the thief has been arrested before.

"In Barcelona you get fined more for double-parking or wandering around with your shirt off than for stealing three purses in one day."

Families with small children, pushchairs and lots of suitcases were favourite targets, she said. "Who is going to chase a pickpocket if they have to worry about their child?"

The thieves not only threaten her, but have sometimes tried to buy her off. "I am very careful about my own security and don't tell anyone where I live," says Guerrero, who moved to Barcelona from her native Colombia 12 years ago.

She has become something of a celebrity in Barcelona, and has challenged the city mayor, Xavier Trias, to ride the metro with her.

"Politicians and judges don't use the metro, so they don't understand what is going on," she said. "We need more plainclothes police down there, but what really needs changing is the law."

She said she would like police not in uniform to take a trip with her.

"And I am sure we would see at least one group of bag-snatchers. Every time I go with a film crew we always find some, but whoever has the camera needs to be fast – because I go like lightning."