President Roch Kabore said gunmen killed about 100 people in an attack on a village in northern Burkina Faso.
Reuters quoted a government statement as saying that during the night-time raid on the Al-Salhan area, houses and a market were burnt.
No group has claimed responsibility for the violence.
But Islamist attacks are increasingly common in the country, particularly in regions bordering Niger and Mali.
President Kabore declared three days of national mourning “We must stand united against the forces of evil,” he tweeted.
He added that the security forces are currently searching for the perpetrators.
In another attack Friday night, 14 people were reportedly killed in the village of Tadariat, about 150 kilometers (93 miles) north of Salhan.
Last month, 30 people were killed in an attack in eastern Burkina Faso.
Burkina Faso is facing a deep security crisis, like many of its neighbours, with armed groups conducting raids and kidnappings across much of the region.
The semi-arid Sahel region of Africa has been rocked by an insurgency since militants seized large parts of northern Mali in 2012 and 2013.
French forces are backed by forces from Mali, Chad, Mauritania, Niger and Burkina Faso to fight the militants.
But this week France has stopped cooperating with Mali On the last coup there.