Zambian Vice-President Guy Scott has been named acting leader following the death of President Michael Sata.
Presidential elections to choose a permanent successor will be held within 90 days, Defence Minister Edgar Lungu said.
Mr Scott becomes Africa's first white head of state since FW de Klerk in apartheid South Africa.
Mr Sata died in the UK aged 77 after receiving treatment for an undisclosed illness.
He was being treated at London's King Edward VII hospital where he died on Tuesday night.
Mr Scott regularly stood in for the president at official events, but was never appointed acting president when Mr Sata was abroad - so this is his first time to officially lead the country.
In a brief televised address Mr Scott confirmed his appointment.
"The period of national mourning will start today. We will miss our beloved president and comrade," Reuters news agency quotes him as saying.
The president's death comes just days after Zambia celebrated the 50th anniversary of independence from the UK.
Cabinet secretary Roland Msiska said on national TV that President Sata's wife and son were at his bedside.
He is the second Zambian leader to die in office after Levy Mwanawasa in 2008.
Earlier this month reports in Zambia said that President Sata had gone abroad for a medical check-up amid persistent speculation that he was seriously ill.
He had rarely been seen in public since returning from the UN General Assembly last month, where he failed to make a scheduled speech.
After he left the country, Defence Minister Edgar Lungu was named as acting president.
Mr Scott is of Scottish descent and his parents were not born in Zambia, so he may not be able to run for president in January because a constitutional clause on parentage could nullify his candidacy.