Historically, after World War II and the era of industrialisation under socialism, the city and its surrounding areas expanded rapidly and became the most heavily industrialised region of the country, with numerous factories producing steel, pig iron, machinery, industrial equipment, electronics, trams, chemicals, textiles, food. The influx of workers from other parts of the country became so intense that a restriction policy was imposed, and residing in the capital was only possible after obtaining Sofianite citizenship. However, after the political changes in 1989, this kind of citizenship was removed.