The noises turned out to be firecrackers, according to the AP.
But the collective panic, captured on video, shows how tense Paris remains two days after ISIS-backed terrorists carried out bombings and shootings across the French capital, killing at least 129 people and injuring more than 350.
Hundreds of people had gathered in the Parisian square, near where much of Friday's violence took place, to hold vigil.
Sudden popping noises sent them running, while police reportedly drew their guns and told people to evacuate.
Here's that moment, via a tweet from the BBC's Julia Macfarlane:
The terrifying moment that a false alarm panicked crowds at the Place de la Republique in central #Paris (Reuters) pic.twitter.com/k31qlv82e3
- Julia Macfarlane (@juliamacfarlane) November 15, 2015
Seven of Friday's attackers have died but an eighth suspect, 26-year-old Salah Abdeslam, remains at large.