The Eiffel Tower was closed to visitors on Wednesday after workers went on strike ahead of contract negotiations with the city of Paris. The walkout coincided with planned events marking the 100th anniversary of the death of the tower’s creator, Gustave Eiffel.

The tower is open 365 days a year, apart from during occasional strikes, and it was unclear when it would open again, an Eiffel Tower spokesperson told The Associated Press.

Around 800 employees work at the tower every day and 6,000,000 people visit it each year, making it the most visited ticketed monument in the world, according to the group that runs the monument, SETE.

A prerecorded musical program marking Gustave Eiffel’s death —  he died on December 27, 1923 — was still scheduled to run on French television on Wednesday night.

The union said in a statement that tower staffers were protesting “the current way it is managed” and said its operator was “headed for disaster,” the AFP news agency reported.