At least three people have died and about 20 others are in hospital in Liberia after a vehicle rammed into supporters of President-elect Joseph Boakai, a medical official has said.
The police described the incident as an “accident”, but Mr Boakai’s party called it an “act of terrorism”.
It came just hours after Liberia’s electoral body declared him the winner of last week’s presidential election.
In his concession speech, President George Weah had called for unity.
It was a closely fought election, with just over 20,000 votes separating the two candidates in the run-off poll.
On Monday evening, Mr Boakai’s supporters were celebrating the official announcement of his victory outside his Unity Party (UP) headquarters in the capital, Monrovia.
There are conflicting reports on the time of the incident, but witnesses place it at some point in the two hours after 21:00 local time (21:00 GMT).
“[The vehicle] just picked up with speed and ran into the supporters,” UP spokesman Mohammed Ali told the AFP news agency.
The driver is reported to have abandoned the vehicle near the party offices before fleeing. The police have launched a manhunt.
Angry onlookers set the vehicle on fire.
Eyewitnesses told the Liberian Observer newspaper that they initially thought the incident was caused by a brake failure, but the driver’s sudden take-off fuelled suspicions of a planned attack.
“We did not hear anything because the music was playing… I don’t really know where the car came from. I just found myself on the ground,” Bendu Kiadii, one of those injured, told the Reuters news agency in a hospital trauma ward.
In a statement, the police called what happened “an accident incident” and urged UP supporters to “remain calm” while investigations were under way.
Dr Sia Camanor, the chief medical officer at Monrovia’s John F. Kennedy Memorial Hospital, said that three people had died of their injuries, while 18 are still being treated. Some are in a critical condition.
Dr Camanor appealed to the public to come forward to donate blood, as some of the victims had lost a lot of it.
She added that four other people had been taken to another hospital for treatment, but she did not have details of their condition.
In an earlier statement, the UP called what happened a “devastating, wicked and barbaric act of domestic terrorism, leading to loss of precious lives”.
The incident followed an election that was deemed largely peaceful, save for some isolated violence in four provinces.
The UP said it was cancelling all celebratory gatherings until further notice.