By Aleksandar Vasovic and Milan Pavicic

BELGRADE, April 2 (Reuters) - Tamara Bojanovski was in a crowd of anti-government protesters in Belgrade on March 15 when she heard a sound "like some powerful machine hurtling up from behind".

Thousands of others heard it too; the crowd packed into one of the Serbian capital's main boulevards parted abruptly, rushing to the sidewalks.

Stefan, a student, recalled a "rumble", then a "whoosh" and a sensation of something speeding toward the crowd. Another student, Dragica, felt "a wave travelling through us".

"People felt faint, and some fell over," said lawyer Bozo Prelevic, a former joint interior minister.

The noise lasted only a few seconds.

But speculation that a sonic weapon was used illegally to disperse the rally has filled headlines, talk shows and social media. President Aleksandar Vucic, already facing the biggest civil protests in decades, is under pressure to explain the incident.

Sonic weapons employ extreme sound to incapacitate targets. They can damage ears and cause headaches and nausea, and their use is illegal in Serbia.

Authorities denied possessing such devices, until Interior Minister Ivica Dacic admitted that police had bought Long-Range Acoustic Devices - used by authorities in the United States, Australia, Greece and Japan - from the U.S. in 2021.

Then Serbia's police, BIA security and intelligence agency and military all denied ever using them in public.

Vucic said on Saturday that Russia had sent experts from its FSB intelligence service to investigate at Belgrade's request, and on Monday said American FBI investigators would also arrive within days. The U.S. Department of Justice did not respond to a request for comment.

The Omega Foundation, a human rights watchdog, said photos and witness accounts they reviewed and audiovisual footage obtained by Reuters were inconclusive, but suggested an LRAD could have been used.

"We really haven't seen an effect like this. It was so distinctive," said Omega Foundation researcher Neil Corney.

Earshot, a not-for-profit organisation that specialises in audio investigations, which also saw the footage, said the noise could have come from a vortex ring gun, an experimental non-lethal weapon for crowd control that uses high-energy doughnut-shaped vortices of air or gas, but that more research was needed.

However, U.S.-based Genasys, which makes LRADs, said that audio and video evidence "does not support the use of an LRAD".

The protesters had gathered in memory of 15 people who died when a train station roof collapsed in November in the city of Novi Sad.

That tragedy, which many blame on government corruption and shoddy construction, has drawn hundreds of thousands onto the streets and forced prime minister Milos Vucevic to resign, as well as continuing to put pressure on Vucic.

Geolocation of the videos suggests that the sound wave travelled south along Kralja Milana Street for over 500 metres.

"The street emptied ... like when Moses parted the Red Sea," said Zoran Radovanovic, an epidemiologist who was in the crowd.

(Reporting by Aleksandar Vasovic, Milan Pavicic; Editing by Edward McAllister and Kevin Liffey)

((aleksandar.vasovic@thomsonreuters.com; +381113044904;))