(Corrects paragraph 13 to make clear that the tower was built for the 1889 World Fair and marked the centenary of the French Revolution)

By Brian Love and Emmanuel Jarry

PARIS, May 5 (Reuters) - Security was stepped up at the Eiffel Tower on Friday ahead of Sunday's presidential election in France after Greenpeace activists scaled the Paris landmark and hung out a big political banner.

The early morning escapade in which activists used mountaineering helmets, ropes and shackles prompted the Paris police chief to call an emergency meeting where the new security measures were decided.

Separately, police officers arrested a "radicalised" former soldier near a military airbase near the town of Evreux northwest of the French capital in an incident that judicial sources said was related to a counter-terrorist inquiry.

A shotgun was found in nearby undergrowth, in addition to a cartridge and a Koran in the car he had left at the edge of the airbase, where he was arrested in the early morning on returning to the spot, the sources said.

Security chiefs, after attacks by Islamist militants, have already announced extra police for patrolling key areas on voting day on Sunday when a centrist politician meets a veteran from the far-right after a turbulent and at times angry campaign.

More than 230 people have been killed in such attacks in Paris and other parts of France in the past two-and-a-half years.

Just three days before a first round of voting on April 23, a policeman was shot dead in central Paris by a gunman in an attack claimed by Islamic state.

A dozen activists from the Greenpeace advocacy group were arrested on Friday after climbing the north face of the vast metal-latticed Eiffel Tower, one of the world's most visited sites, to hang a banner carrying the French national motto, "Liberte, Egalite, Fraternite" (Liberty, Equality, Fraternity).

"Above and beyond the motives, this publicity stunt, in the current climate, exposes faults in the security arrangements at the Eiffel Tower," police prefect Michel Delpuech said.

Paris City Hall issued a statement after an emergency meeting to say sniffer-dog patrols would be more than doubled, and video-monitoring around the area improved.

As tension mounted ahead of Sunday's vote, police arrested a man pre-dawn when he returned to his car parked near a military airbase in Evreux, judicial sources said.

ANTI-NATIONALISM PROTEST

At least three in four of 47 million voters are set to go to polling stations throughout the day on Sunday to cast a ballot in a contest where centrist Emmanuel Macron is tipped to beat National Front leader Marine Le Pen.

The Paris crisis talks involved police as well as officials from City Hall and the company that runs the tower which was built for the World Fair in 1889 and marked the centenary of the French Revolution.

It is visited by an average of about 19,000 people per day.

A dozen Greenpeace activists were detained for questioning, police said.

Beneath the French republican slogan in large black letters was the word 'Resist', a message directed against Le Pen and her National Front party.

Greenpeace said a dozen activists were involved, hoisting a 300-square-metre banner at around 7.45 a.m, which is before normal business hours but well after dawn has broken at this time of year.

"We wanted to say we are against the rise of nationalism and authoritarianism in France and in other countries," Greenpeace France's head, Jean-Francois Julliard, added on public radio station francinfo.

(Additional reporting by Emmanuel Jarry, Andrew Callus and John Irish; Writing by Brian Love; Editing by Richard Balmforth)

Our Standards: The Thomson Reuters Trust Principles.