The plan to build a new footbridge next to the Mont-Blanc bridge could be scuppered by a referendum.
The CHF 40m project has been passed by the city council. But some consider the idea to be unnecessary and too expensive.
Campaigners now have less than a month to collect 3,200 signatures.
Despite the threat of a referendum, work has started. The city has already launched a call for tenders for wooden piles needed for the construction, valued at half a million francs.