Mozambique's opposition Renamo movement has ended a 1992 peace accord after government forces attacked the base of its leader, Afonso Dhlakama.
The government forces captured the Sathunjira base in central Mozambique, forcing Mr Dhlakama to flee.
About a million people were killed in the civil war that raged in Mozambique after it achieved independence from Portugal in 1975.
Mozambique's economy has been booming since the civil war ended.
Renamo spokesman Fernando Mazanga said that government soldiers had bombarded the Sathunjira base in central Sofala province with heavy weapons before occupying it on Monday.
Force of hundreds
"Peace is over in the country... The responsibility lies with the Frelimo government because they didn't want to listen to Renamo's grievances," Mr Mazanga told Reuters news agency.
The attack was an attempt to assassinate Mr Dhlakama but he managed to escape to an undisclosed location, Mr Mazanga said.
In a statement, Renamo blamed President Armando Guebuza for the attack.
"This irresponsible attitude of the commander-in-chief of the defence and security forces puts an end to the Rome peace deal," it said.
The BBC's Jose Tembe in the capital, Maputo, says Renamo's statement suggests that it plans to go back to war, but it has denied this in the past.
Defence ministry spokesman Cristovao Chume said government forces had taken control of the base in response to an earlier attack on an army post by Renamo fighters.
He confirmed that Mr Dhlakama had fled.
Mr Chume and Mr Mazanga did not give any casualty figures.
Mozambique's Frelimo government has repeatedly accused Renamo of dragging the country back to war, an allegation it denies.
In April, at least five people were killed in central Mozambique after Renamo members attacked a police post.
A force of about 300 Renamo men has remained armed since the peace accord, despite efforts to integrate them into the army or police force.
Mr Dhlakama has said he needs his own personal bodyguards, and the men usually stay in his bush camp in the Gorongosa mountains.
After the civil war ended, Mr Dhlakama moved out of the camp to live in Maputo and later in the northern Nampula province
But he returned to the mountains last year, saying he needed to be close to his men who were feeling ignored.
Mozambique is due to hold local elections in November, and presidential and parliamentary elections next year.
Mr Guebuza's Frelimo party has governed Mozambique since independence in 1975.
Renamo, which was formed around the same time, was backed by the white minority governments in neighbouring South Africa and what is now Zimbabwe.