and Tutsi different races. In 1935, Belgium introduced an identity card system, which labelled each individual as either Tutsi, Hutu, Twa or Naturalised. While it had been previously possible for particularly wealthy Hutu to become honorary Tutsi, the identity cards prevented any further movement between the classes. Belgium continued to rule Ruanda-Urundi (of which Rwanda formed the northern part) as a UN trust territory after the Second World War, with a mandate to oversee eventual independence. Tensions escalated between the Tutsi, who favoured early independence, and the Hutu emancipation movement, culminating in the 1959 Rwandan Revolution: Hutu activists began killing Tutsi and destroying their houses, forcing more than 100,000 people to seek refuge in neighbouring countries. In 1961, the suddenly pro-Hutu Belgians held a referendum in which the country voted to abolish the monarchy. Rwanda was separated from Burundi and gained independence on 1 July 1962, which is commemorated as Independence Day, a national holiday. Cycles of violence followed, with exiled Tutsi attacking from neighbouring countries and the Hutu retaliating with large-scale slaughter and repression of the Tutsi. In 1973, Juvénal Habyarimana took power in a military coup. Pro-Hutu discrimination continued, but there was greater economic prosperity and a reduced amount of violence against Tutsi. The Twa remained marginalised, and by 1990 were almost entirely forced out of the forests by the government; many became beggars. Rwanda's population had increased from 1.6 million people in 1934 to 7.1 million in 1989, leading to competition for land. In 1990, the Rwandan Patriotic Front (RPF), a rebel group composed of nearly 500,000 Tutsi refugees, invaded northern Rwanda from their base in Uganda, initiating the Rwandan Civil War. The group condemned the Hutu-dominated government for failing to democratize and confront the problems facing these refugees. Neither side was able to gain a decisive advantage in the war, but by 1992 it had weakened Habyarimana's authority; mass demonstrations forced him into a coalition with the domestic opposition and eventually to sign the 1993 Arusha Accords with the RPF. The cease-fire ended on 6 April 1994 when Habyarimana's plane was shot down near Kigali Airport, killing him. The shooting down of the plane served as the catalyst for the Rwandan genocide, which began within a few hours. Over the course of approximately 100 days, between 500,000 and 1,000,000 Tutsi and politically moderate Hutu were killed in well-planned attacks on the orders of the interim government.