The Eiffel Tower has been closed to visitors after a terror suspect carrying a large rucksack was reportedly spotted scaling the attraction.
A helicopter could be seen circling the 986-foot structure after officials raised the alarm just before 9am local time, according to The Daily Mail.
According to the newspaper, a police source helping evacuate the tower said: "There's been a report of a man with a rucksack."
Hundreds of people queueing to get into the attraction were reportedly turned away, with a police cordon formed around its base.
The suspect had not been found by early afternoon.
A number of terror groups, including Isis and al-Qaeda, have threatened to target the Eiffel Tower - which is France's most popular visitor attraction.
Security fears have been heightened in Paris since deadly attacks on satirical magazine Charlie Hebdo in January, which saw 12 people killed.
In August, passengers subdued a gunman apparently attempting to carry out a mass shooting on a train from Amsterdam to Paris.