the new capital was inaugurated on 13 February 1931. New Delhi was officially declared as the capital of the Union of India after the country gained independence on 15 August 1947. It has expanded since; the small part of it that was constructed during the British period has come to be informally known as Lutyens' Delhi. Partition and post-independence During the partition of India, around five hundred thousand Hindu and Sikh refugees, mainly from West Punjab fled to Delhi, while around three hundred thousand Muslim residents of the city migrated to Pakistan. Ethnic Punjabis are believed to account for at least 40% of Delhi's total population and are predominantly Hindi-speaking Punjabi Hindus. Migration to Delhi from the rest of India continues (), contributing more to the rise of Delhi's population than the birth rate, which is declining. The States Reorganisation Act, 1956 created the Union Territory of Delhi from its predecessor, the Chief Commissioner's Province of Delhi. The Constitution (Sixty-ninth Amendment) Act, 1991 declared the Union Territory of Delhi to be formally known as the National Capital Territory of Delhi. The Act gave Delhi its legislative assembly along Civil lines, though with limited powers. Delhi was the primary site in the nationwide anti-Sikh pogroms of 1984, which resulted in the death of around 2,800 people in the city according to government figures, though independent estimates of the number of people killed tend to be higher. The riots were set off by the assassination of Indira Gandhi—the Prime Minister of India at the time—by her Sikh bodyguards. In 2001, the Parliament of India building in New Delhi was attacked by armed militants, killing six security personnel. India suspected Pakistan-based Jihadist militant groups were behind the attack, which caused a major diplomatic crisis between the two countries. There were further terrorist attacks in Delhi in 2005 and 2008, resulting in a total of 92 deaths. The 2020 Delhi riots, Delhi's worst communal violence in decades, was caused mainly by Hindu mobs attacking Muslims. Of the 53 people killed, two-thirds were Muslims, and the rest Hindus. Geography Delhi is located in Northern India, at . The city is bordered on its northern, western, and southern sides by the state of Haryana and to the east by that of Uttar Pradesh (UP). Two prominent features of the geography of Delhi are the Yamuna flood plains and the Delhi ridge. The Yamuna River was the