More than half a million people have signed an online petition calling for an independent investigation into whether security forces in Serbia used a sonic weapon – what the petition described as a “sound cannon” – during Saturday’s huge anti-corruption rally.

Days after as many as 325,000 people took to the streets of Belgrade, rights groups and opposition parties continue to allege that protesters were targeted with some sort of auditory device that briefly sowed panic and left some with symptoms that lingered long after the rally.

Serbian politicians and police have denied the allegations. Earlier this week, the country’s increasingly autocratic president, Aleksandar Vučić, described the claim as a “wicked lie” that was aimed at “destroying Serbia”.