The veteran leader, who seized power in 1965 but has seen his hold over the country weakened by relentless rebel advances, announced the move on state television.
President Mobutu said a state of emergency declared in the eastern provinces of North and South Kivu last year "is extended throughout the national territory".
The announcement came after Zaire's warring sides completed peace talks in South Africa yesterday during which they agreed on the need for a ceasefire but gave no details of how to implement it.
Delegates representing President Mobutu and the rebel leader Laurent Kabila adjourned their negotiations for four days after meeting for the first time in Pretoria.
"Both parties agreed on negotiations to bring about a peaceful, political solution to the conflict," a joint statement said.
"This necessitates a complete cessation of hostilities and other measures consistent with the implementation of the UN-OAU peace plan," said the statement.
Mr Kabila said yesterday that rebel "elements" had reached a town only 170 miles north-east of the capital, Kinshasa. "We are in Kasai Oriental. Elements have arrived in Bandundu and Mbandaka," Mr Kabila said. Kasai Oriental is the eastern part of Kasai region with Mbuji-Mayi as its capital.
Soldiers in Zaire's collapsing army said they had driven back a rebel advance on the copper city of Lubumbashi, which was rocked by explosions overnight. "We have repulsed the enemy," said an officer from President Mobutu's Special Presidential guard (DSP), the only part of the army which seems ready to put up a fight. Lubumbashi, Zaire's second city, is the next declared target of the rebels.
The DSP has more weapons and is better disciplined than the rest of Zaire's ragged army that has distinguished itself in looting during the civil war but has put up little resistance to the rebels. Its defined role is to protect Mr Mobutu and vital national interests.