The tower was reopened at 2:45 pm (1245 GMT) after being closed to the public for the morning, the press department for the tourist attraction said.
Before it opened on Sunday morning, "an individual was seen in the structure with a backpack", a source close to the matter said, speaking on condition of anonymity.
"To remove all doubt, a helicopter was sent to carry out an inspection of the structure."
Firefighters were also dispatched to the site, but "found nothing", according to a fire department spokesman.
A policeman at the scene said the intruder may have been an adventurer seeking to parachute off the tower, which is 324 metres (354 yards) tall -- a sight not uncommon in the French capital. In 2005, however, a Norwegian man died while attempting a similar feat.
The "Iron Lady" attracts around seven million visitors a year, of which nearly nine in 10 are foreign tourists.
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