During its early years, as '''Örgöö''' (anglicised as '''Urga'''), it became Mongolia's preeminent religious centre and seat of the Jebtsundamba Khutuktu, the spiritual head of the Gelug lineage of Tibetan Buddhism in Mongolia. Following the regulation of Qing-Russian trade by the Treaty of Kyakhta in 1727, a caravan route between Beijing and Kyakhta opened up, along which the city was eventually settled. With the collapse of the Qing Empire in 1911, the city was a focal point for independence efforts, leading to the proclamation of the Bogd Khanate in 1911 led by the 8th Jebtsundamba Khutughtu, and again during the communist revolution of 1921. With the proclamation of the Mongolian People's Republic in 1924, the city was officially renamed Ulaanbaatar and declared the country's capital. Modern urban planning began in the 1950s, with most of the old ger districts replaced by Soviet-style flats. In 1990, Ulaanbaatar was a major site of demonstrations that led to Mongolia's transition to democracy and a market economy. Since 1990, an influx of migrants from the rest of the country has led to an explosive growth in its population, a major portion of which live in ger districts, which has led to harmful air pollution in winter.