PARIS (Reuters) - More than a thousand police and firemen took part in the largest attack response drill ever carried out in France this week in the southern city of Nîmes, ahead of the Euro 2016 football championship due to be held in the country from June 10.

French authorities are keen to show they are well prepared for the event, which will take place less than a year after Islamist attacks in Paris that killed 130 people.

The exercise was designed to simulate a chemical attack in a "fan zone", a closed perimeter where football fans will be able to monitor the competition on giant outdoor screens when not attending matches in one of the 10 stadiums.

A Reuters witness saw dozens of police and military officers taking part in the drill, some of them equipped with gas masks.

French anti-terrorism police arrested a group with Islamist militant ties on Wednesday, suspecting one of them may have been planning another attack in the capital.

"In Nîmes, it was about a chemical or bacteriological threat," Interior Minister Bernard Cazeneuve was quoted as saying in daily newspaper 20 minutes. "We don't believe there is a genuine risk of this type of attack but we must envisage every hypothesis."

It will be the third time that France hosted the UEFA European Championship after having been chosen for the 1960 inaugural tournament and the finals in 1984.