Three days of mourning have been declared in Ivory Coast following a New Year's Eve stampede that killed at least 61 people in Abidjan. Hundreds of people leaving a fireworks event at a stadium in the Plateau district were jammed into a tiny street in the early hours of Tuesday: many were crushed; others suffocated. An investigation is under way. Many victims were said to be 15 or younger. President Alassane Ouattara described the deaths as a national tragedy. Different theories are emerging as to what caused the stampede after the fireworks display ended in the 65,000-capacity Felix Houphouet-Boigny Stadium. 'Trampled with our children' Some say a group of youths brandishing knives was snatching people's mobile phones, causing panic among the large crowd, says the BBC's West Africa correspondent Thomas Fessy. But others say security forces acted ineptly as they tried to control the many thousands walking through the city centre - triggering the stampede which has left many more people injured, some critically. Desperate parents went to the city morgue, hospital and stadium to search for their children, AP news agency reports. They included Mamadou Sanogo, who was looking for his nine-year-old son, Sayed, it adds. "I have just seen all the bodies, but I cannot find my son. I don't know what to do," a tearful Mr Sanogo said. A market vendor injured in the incident, Sanata Zoure, told Reuters news agency that revellers going home after watching the fireworks display were stopped by police near the stadium. "We were walking with our children and we came upon barricades, and people started falling into each other. We were trampled with our children," she is quoted as saying. Mr Ouattara visited some of the wounded who were taken to two hospitals in the country's main city and promised that the costs of their medical treatment would be covered by the government. The fireworks event had been organised to celebrate the end of the conflict triggered by former President Laurent Gbagbo's refusal to recognise Mr Ouattara's victory in 2010 elections. Mr Gbagbo is now facing trial on war crimes charge at the International Criminal Court in The Hague. The night before the tragedy, the stadium, named after Ivory Coast's founding president, hosted a concert by US performer Chris Brown. In 2009, 19 football fans died during a crush at the same stadium before a World Cup qualifier against Malawi.