President Macky Sall tweeted that the collision happened in Gnivy village, in the Kaffrine region, at about 3:30 a.m.
“I am deeply saddened by the tragic road accident today in Gniby causing 40 deaths and many serious injuries. I extend my heartfelt condolences to the families of the victims and wish a speedy recovery to the injured,” said Sall.
He declared three days of mourning starting Monday and said he will hold an inter-ministerial council to discuss road safety measures, AP reported.
Public prosecutor Cheikh Dieng said the crash happened on National Road No. 1 when a public bus punctured a tire and veered across the road, colliding with another bus coming from the opposite direction. At least 78 people are injured, some of them seriously, he said.
Images of the crash on social media show the damaged buses rammed into each other and a trail of debris along the road.
Traffic crashes happen regularly in the West African nation because of poor roads, bad cars and drivers not adhering to the rules, locals say.
In 2017, at least 25 people were killed when two buses also crashed. Many of those people were heading toward the central town of Touba for the annual Muslim pilgrimage.