Update: man wanted to attack soldier
The man has told investigators that he wanted to attack a soldier and was in touch with a member of the Islamic State armed group who encouraged him to act, sources have told the AFP news agency.
The man, who was apparently alone, was "very quickly overpowered and arrested" when he tried to force his way through barriers at the entrance to the Eiffel Tower, a statement by the company that operates the monument said.
Nobody was hurt.
The incident happened at 11.30pm Saturday night, about half an hour before the tower normally stops admitting visitors.
The man, who was wearing a Paris St-Germain football shirt, barged past a security guard and then took out a knife, shouting "Allahu Akbar!", sources said Sunday.
The man, a French national born in Mauritania in 1998, left a psychiatric hospital in July, they said.
An investigation into justifying terrorism and attempted murder has been opened.
After the arrest, police checked the site and called for it to be evacuated at around 00.30, 15 minutes before it usually closes, the statement added.
At the time of the incident the tower was lit up with the team colours of PSG to celebrate Neymar's signing by the club this week.
A screen carried a message welcoming the world's most expensive footballer to France.
The cost of the display was born by PSG.
France has been under a state of emergency since the November 2015 Paris attacks.
The Eiffel Tower opened as normal on Sunday at 9.00am.