Mozambique's opposition Renamo movement has attacked a police station in the central town of Maringue a day after annulling a 1992 peace accord with the government, officials say.
Police are said to have fled their posts after gunmen opened fire and there are no reports of casualties.
A Renamo spokesman said party leader Afonso Dhlakama had "lost control of the situation".
On Monday, the army captured Mr Dhlakama's base, forcing him to flee.
About a million people were killed in Mozambique's civil war.
The conflict ended in 1992 when Renamo and the Frelimo government signed a peace accord.
After the seizure of Mr Dhlakama's Sathunjira base on Monday, Renamo said it had pulled out of the peace deal.
However, its 51 MPs have not withdrawn from parliament.
The clashes have raised fears among many Mozambicans that the civil war could resume, threatening stability and the economic boom the southern Africa state is currently experiencing, correspondents say.
State-run Radio Mozambique quoted a police source as saying that Renamo fighters attacked the police station in Maringue, about 35km (20 miles) from Mr Dhlakama's former base in central Sofala province.
The attack was intense, but there were no casualties, Radio Mozambique reported.
A teacher in Meringue, Romao Martins, told AFP news agency that residents had fled their homes during the assault.
Schools have also shut amid fears that the violence could escalate, AFP reports.
"The situation is horrible here. Early this morning, armed men supposed to be Renamo, attacked and it was a mess," Mr Martins is quoted as saying.
"For one hour, shooting could be heard from all directions and people fled from their homes."
On Monday, defence ministry spokesman Cristovao Chume said government forces had taken control of Mr Dhlakama's base in response to an earlier attack on an army post by Renamo fighters.
He confirmed that Mr Dhlakama had fled.
Renamo spokesman Fernando Mazanga said that government soldiers had bombarded the Sathunjira base with heavy weapons before occupying it.
"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 government has repeatedly accused Renamo of trying to drag 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 war-time camp in the Gorongosa mountains in centre of the country.
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.
Frelimo 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. (BBC)