Addiction haunts the recesses of this ancient port city, as people with gaunt, clumsy hands lift crack pipes to lips, syringes to veins. Authorities are sealing off warren-like alleyways with iron bars and fencing in parks to halt the spread of encampments. A siege mentality is taking root in nearby enclaves of pricey condos and multimillion-euro homes. Portugal decriminalized all drug use, including marijuana, cocaine and heroin, in an experiment that inspired similar efforts elsewhere, but now police are blaming a spike in the number of people who use drugs for a rise in crime. In one neighborhood, state-issued paraphernalia — powder-blue syringe caps, packets of citric acid for diluting heroin — litters sidewalks outside an elementary school. Porto’s police have increased patrols to drug-plagued neighborhoods.

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