The Kingdom of Cambodia is a country in the form of a constitutional monarchy in Southeast Asia. This country is the successor of the Khmer Empire which once controlled the entire Indochina Peninsula between the 11th and 14th centuries. The sovereign state of Cambodia has a population of over 15 million. Buddhism is enshrined in the constitution as the official state religion, and is practiced by more than 97% of the population. Cambodia's minority groups include Vietnamese, Chinese, Chams and 30 hill tribes. The capital and largest city is Phnom Penh, the political, economic and cultural center of Cambodia. The kingdom is an elective constitutional monarchy with a monarch, currently Norodom Sihamoni, chosen by the Royal Council of the Throne as head of state. The head of government is the Prime Minister, currently Hun Sen, the longest serving non-royal leader in Southeast Asia, ruling Cambodia since 1985.