President Francois Hollande says French troops are taking part in operations against Islamists in northern Mali.
French troops "have brought support this afternoon to Malian units to fight against terrorist elements", he said.
Armed groups, some linked to al-Qaeda, took control of northern Mali in April.
Mr Hollande said the intervention complied with international law, and had been agreed with Malian President Dioncounda Traore. A state of emergency has been declared across the country.
Mr Traore used a televised address on Friday to call on Malians to unite to "free every inch" of the country.
He said he was to launch a "powerful and massive riposte against our enemies" after he "called for and obtained France's air support within the framework of the international legality".
The militants said on Thursday that they had advanced further into government-controlled territory, taking the strategic central town of Konna.
The Islamists have sought to enforce an extreme interpretation of Islamic law.
Residents in nearby Mopti told the BBC they had seen French troops helping Malian forces prepare for a counter-offensive against the Islamists in Konna.
Mr Hollande said French military action had been decided on Friday morning and would last "as long as necessary".
"Mali is facing an assault by terrorist elements coming from the north whose brutality and fanaticism is known across the world," the French president said.
He said Mali's existence as a state was under threat, and referred to the need to protect its own population and 6,000 French citizens living there. France ruled Mali as a colony until 1960.
French Foreign Minister Laurent Fabius said the aim of the operation was to stop Islamist militants advancing any further.
"We need to stop the terrorists' breakthrough, otherwise the whole of Mali will fall into their hands threatening all of Africa, and even Europe," he told reporters.
He confirmed that the French air force was involved in the operation, but gave no details.
France was previously believed to have about 100 elite troops in the region. It also has a military base in Chad.
At least seven French hostages are currently being held in the region, and Mr Fabius said France would "do everything" to save them.
British Foreign Secretary William Hague said on Twitter that the UK supported the French decision to help Mali's government against northern rebels.
The US and African Union have also expressed support for the mission.
Shortly after Mr Hollande spoke, the west African bloc Ecowas said it was authorising the immediate deployment of troops to Mali "to help the Malian army defend its territorial integrity", AFP news agency reported.
The Malian army said that as well as French troops, soldiers from Nigeria and Senegal were already in Mali - though Senegal later denied that it had any combat troops in the country, according to AFP.
The UN had previously approved plans to send some 3,000 African troops to Mali to recapture the north if no political solution could be found, but that intervention was not expected to happen until September.
Late on Thursday, an emergency meeting of the UN Security Council called for the rapid deployment of an African-led force.
The EU also said it would support such a mission, and speed up preparations for its own military training mission.