The French town of Nimes, where the first stage of the Vuelta a España was held today, is on high alert and the train station has been evacuated.
One man has been arrested, the local newspaper Midi Libre reported.
Another security official said police are working to verify whether the incident had any link to the attacks Thursday around Barcelona. It said reports that shots had been fired were incorrect, but police were looking for a second man.
Local police have advised people to steer clear of the area surrounding the train station where gunshots were reportedly heard and which has been evacuated.
While law enforcement agencies at present say the reports were unfounded, one person has been detained for "other reasons" following "suspicious activity".
The men boarded the train at Nimes, just east of Spain.
Unverified reports stated that up to three suspects may have entered the train station and open fired.
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