Ivory Coast declared a three-day national period of mourning after a New Year’s Eve stampede killed at least 60 people in Abidjan.

Several theories have emerged explaining the possible reasons for the tragedy that occurred when hundreds of people were leaving a fireworks display at a stadium in the Plateau district.

Some say a group of youths was robbing people's mobile phones at knifepoint, provoking panic among the large crowd, the BBC reported.

But others blame the deaths on the inept handling of the situation by the security forces who tried to control the crowd that was walking through the city center, triggering the stampede.

Approximately 50,000 people gathered for the fireworks, according to officials.

Many victims were said to be 15 or younger. The government said 60 people with an average age of 18 had died.  In addition to the deaths, 49 others were injured, two seriously.

President Alassane Ouattara described the deaths as a national tragedy.

“This is a real tragedy on this New Year's Day," Ouattara said at the scene.

"We are all in shock," he added, declaring three days of national mourning starting Wednesday.

Ouattara visited some of the wounded, who were taken to two hospitals in the country's main city and promised that the government would bear the costs of their medical treatment.

"The president of the republic offers his saddest condolences to the families and close relations of the victims and ensures them of his compassion in those painful circumstances,” Ouattara’s office said in a statement issued Tuesday night.

An investigation was under way to determine the “exact circumstances” of the tragedy, Interior Minister Hamed Bakayoko was quoted as saying by Reuters. He said the stampede happened as hundreds of people were trying to go home after the fireworks ended, adding that proper security measures were in place during the event.

Many of the victims were trampled on or suffocated by the surging crowd, a senior fire official said on the national television, according to a CNN report. Rescue workers reached the scene two hours later but could not save the victims, the official AIP news agency said.

The fireworks event was organized to mark the return of peace after a period of unrest triggered by former President Laurent Gbagbo's refusal to recognize Ouattara's victory in the 2010 elections.

American singer Chris Brown had performed a concert at the stadium the night before the tragedy.

Eighteen people were killed in a stampede during a football match in an Abidjan stadium in 2009.