The Basque separatist group ETA has handed over several cases of weapons and munitions to French authorities as a confidence-building measure ahead of its expected disbanding.
French prosecutor Samuel Vuelta Simon said he had received a tip-off that "a quantity of arms" belonging to ETA had been retrieved by police after being dropped at a location in the south of the French Basque city.
A source close to the case said the police had found four cases "in the open air, in the city centre" containing around 20 handguns, several hundred bullets for 9mm and .38-calibre revolvers and a 20-metre-long detonator cable as well as several electronic detonators.
They also found around 200 fake registration plates and materials that can be used to steal cars, added the source.
ETA last year had already provided French authorities with a list of eight arms caches.
A member of a group mediating in the peace process said that ETA had committed to finding those weapons "which had gone astray" and not been included on the list.
The announcement comes days before ETA is expected to announce its dissolution.
This would bring to a end its nearly four-decade campaign of bombings and shootings for an independent Basque state in northern Spain and southwest France.
Last week it asked forgiveness from its victims for the "pain" caused by its campaign.
Created in 1959 at the height of Francisco Franco's dictatorship ETA is blamed for killing at least 829 people before it announced a permanent ceasefire in 2011.
International mediators are organising a peace conference in southwest France on 4 May.