At least 38 people have died and scores were injured when two buses collided near the town of Kaffrine in central Senegal on Sunday, according to the president and the fire services.
In response to the "grave" accident, President Macky Sall announced three days of national mourning beginning Monday.
In a tweet Sunday he also said there had been 40 deaths and "many serious injuries".
"I am deeply saddened by the tragic road accident," he said on Twitter.
"I extend my heartfelt condolences to the families of the victims and wish a speedy recovery to the injured."
The Senegalese prosecutor said in a separate statement that 38 people had been killed.
Public prosecutor, Cheikh Dieng, said that early investigations suggested that the accident was caused when "a bus assigned to the public transport of passengers, following the bursting of a tyre, left its trajectory before colliding head-on with another bus coming in the opposite direction."
Colonel Cheikh Fall, who is in charge of operations for the West African country's National Fire Brigade, had earlier told AFP that 38 people had died and 87 were injured in the accident.
The accident took place around 3.15 am local time (0315 GMT) Sunday morning, Fall told AFP, adding that all the victims have been evacuated to a hospital and medical centre in Kaffrine.
The wreckage and demolished buses have since been cleared and normal traffic has already resumed on the road, he said.
The governor and local officials have already visited the scene, he added.
President Sall said after the period of national mourning finished, a government council will be held to "take firm measures on road safety".
Road accidents are common in Senegal, largely because of driver indiscipline, poor roads and decrepit vehicles, say experts.
However this is one of the heaviest death tolls from a single incident in recent years.
In October 2020, at least 16 people were killed and 15 more injured when a bus collided with a refrigerated lorry in western Senegal.
The bus, with a 60-seat capacity, was heading to Rosso near the border with Mauritania, the fire brigade said, adding that the number of people onboard was unknown.
Local media said at the time that the truck was hauling fish to Dakar.