The Union Flag is to fly at half mast from Preston Town Hall, following a terrorist attack in Nice.
At least 84 people, including several children, died after a lorry was driven through crowds celebrating Bastille Day in the French city.
A further 50 people are thought to have been injured, 18 critically.
Coun Peter Rankin, leader of Preston Council, condemned the attack as “dreadful”, and said the flag would be flying at half mast until Monday.
Council bosses also announced that the Harris would be lit up in the colours of the French flag during the night throughout the weekend.