(ANSA-AFP) - BELGRADE, APR 8 - Serbian activists delivered a petition with more than half a million signatures to the UN in Belgrade Tuesday demanding an international investigation into the alleged use of sonic weapons against protesters during last month's massive anti-corruption demonstrations. Serbia's largest protest in decades descended into panic, triggered by an unexplained noise. Social media footage showed crowds fleeing, prompting claims that a sonic weapon or "sound cannon" was used on the protesters -- something authorities have strongly denied. Several Serbian civil society groups say they collected over 3,000 testimonies, with some protesters reporting lasting health issues. Some protesters said they heard a noise resembling that of a plane about to crash, while others said it sounded like a car rushing toward them. The petition was launched by the KreniPromeni (Go for Change) movement calling for an independent international investigation into the incident. It also urges that those who ordered, enabled or used such a weapon be held accountable. "The nearly 600,000 people who signed this petition reflect a high level of civic awareness,"movement president Savo Manojlovic told reporters. Photos published after the protest show a police vehicle with a large device mounted on its bonnet. (ANSA-AFP).