More than a dozen people were injured and several are feared dead in Liberia after a vehicle rammed into supporters of President-elect Joseph Boakai.
The police have described the incident as an "accident", but Mr Boakai's Unity Party (UP) called it a "barbaric act of domestic 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 the 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.
In a statement, the police called what happened "an accident incident" and urged UP supporters to "remain calm" while investigations were under way.
It said that 16 people were injured with two unconscious in hospital. UP chairman Luther Tarpeh has said that two people are confirmed dead and four others are "very critical".
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.
