."A climber has been detected. This is the usual procedure,” said the police source, adding that the person has to be prevented from scaling the tower and it has to be evacuated. No indication of the number of tourists evacuated was given.
With firefighters and police in place, the entrance was also evacuated.
The police said they were able to get in touch with the climber, but they were unaware of the reasons behind his actions.
"A climber has been spotted. It's the standard procedure. We have to stop the person, and in that case we evacuate the tower," an official with the SETE operator said.
The esplanade underneath the monument was also evacuated, as was a large section of the adjacent Champ de Mars park.
"We kindly advise our visitors to postpone their visit," SETE added on Twitter.
At around 5:30 pm (1530 GMT) firemen were trying to reach the climber, dressed in a black jacket, by rappelling down from the third-floor observation deck near the top.
Crowds of tourists remained in the area, hoping the tower would eopen soon.
"We're really disappointed, we're only here for a week and this messes with our whole programme," said Sylvie and Celine Forcier from Quebec.
Justin and Karen Smith, from Los Angeles, had celebrated their wedding Sunday in front of the tower, but were hoping to get to the top Monday. "We're disappointed," he said.
The tower is regularly the target of rogue freeclimbers hoping to scale one of the world's most famous structures, often for bragging rights.
The Eiffel Tower in 1889, the year construction was completed and it was opened to the public.
The Eiffel Tower in 1900, at the time of another World Fair. It was slated to be dismantled in 1909, but its use as a communications tower in the early 1990s allowed it to remain standing.
The Eiffel Tower was lit with the colours of the flag of the European Union during the six months that France held the bloc's rotating presidency in 2008.
An estimated 400,000 people watched the firework at a concert of French pop star Johnny Hallyday on 10 June 2000.
The Eiffel Tower in the snow, winter 2010.
The Eiffel Tower during an annual fireworks show to mark France's national holiday, Bastille Day, 14 July 2013.
The Eiffel tower was lit up green with the words "Paris Agreement is Done" after world leaders signed an historial climate deal at the United Nations COP21 conference in the French capital, December 2015.
French soldiers tour the zone around the Eiffel Tower as part of an antiterrorism mission that was reinforced after attacks in 2015 and 2016.
The Eiffel Tower through a small-particle haze as air pollution levels rose in Paris, 23 January 2017.
A couple kiss in front of the Eiffel tower on Valentine's day, 14 February 2015.
But police have also been called in several times in recent years to try to thwart suicide attempts.
In October 2017, a young man ventured out on one of the beams and threatened to jump before police were able to convince him to come back.
In 2012, a British man managed to climb to the very top of the 324-metre-high tower before plunging to his death.
Nearly seven million people a year visit the tower, which last week celebrated its 130th anniversary.
The first two floors can be reached by either elevator or stairs, but only elevators whisk people to the top observation deck.
That did not stop the French urban freeclimber Alain Robert from making it one of his first targets in his campaign to scale the world's biggest buildings with no technical climbing gear.
He got to the top -- not including the antenna-- in the mid-1990s.
The last time the Eiffel Tower was completely evacuated was in October 2017 after a young man standing on one of the pillars was trying to commit suicide. Police were able to convince him not to jump.
⚠️🇬🇧 [Wednesday May 15th / Afternoon] To celebrate my 130th birthday, I am welcoming 1300 children. Thus, a busy period is expected. Choose a visit in the morning or the evening if you did not buy your ticket online.#EiffelTower pic.twitter.com/H7pOJbN5HELa tour Eiffel (@LaTourEiffel) May 13, 2019
The iconic tower celebrated its 130th anniversary this year, and remains the most visited monument worldwide with paid admission some seven million visitors a year.