London - French urban freeclimber Alain Robert was arrested on Thursday after bringing parts of London's financial district to a standstill by scaling the 46-storey Heron Tower - the area's tallest building.

The 56-year-old so-called "French Spiderman" took around an hour to climb the 230m high tower without ropes or safety equipment, while traffic came to a halt as bemused crowds packed the streets below.

Police in the City of London, the capital's square-mile financial centre, confirmed he was then arrested "for causing a public nuisance" and remained in custody Thursday afternoon.

"This is what I love to do," Robert had told a handful of reporters at a nearby hotel shortly before beginning his ascent.

"I have pretty much dedicated my whole life to climb mountains, to climb cliffs, and now to climb buildings - but always 'free soloing', meaning I'm not using safety devices."

The maverick climber has scaled more than 100 structures globally - including several in London previously - setting a record for "most buildings climbed unassisted," according to Guinness World Records.

Earlier this year, he was forced to abandon climbing the world's fifth-tallest tower in Seoul, South Korea, after the 123-storey building's security confronted him halfway up.

Robert admitted Thursday he was "very nervous", adding he always felt anxious ahead of an ascent.

"When I start climbing it's OK, because I know I'm going to be completely focused," he added. 

"But at this point of time I'm a little bit shaky."

The Frenchman, who climbed with a small camera on his forehead, soon attracted hordes of mobile phone-wielding onlookers after starting at lunchtime from busy Bishopsgate on the southwestern side of the building.

Police arrived within minutes and cordoned off nearby roads, quickly clogging traffic in the area. 

"It's weird - that's my office," said 36-year-old finance worker John Doherty, gazing up at Robert as he tackled the lower section of the mixed-use tower, home to offices, restaurants and a bar.

"I've just come back from lunch and it's a surprise."

Robert threw his arms in the air in apparent jubilation after reaching the top before disappearing out of sight.

Prior to the stunt, he said he "definitely expected" to be detained - as had occurred after his six other climbs in the British capital.

"They always arrest me," he added.

"(In) '95 they were nice, and then after (it) started to get more complicated after 9/11 and everything."

Several skyscrapers in London, including the tower at Canary Wharf, have taken out injunctions against Robert to prevent him climbing them again, according to his manager Bryan Yeubrey.

He said Thursday's site was chosen from a shortlist of three, which also included the 224m-high Leadenhall Building - nicknamed "the cheesegrater" - and the 38-story "Walkie-Talkie" tower.

"He just wanted to climb in London again," Yeubrey explained. 

However, City of London Police criticised the stunt for causing "immense disruption to everyday business".

"It also posed a significant level of risk to the safety of people in and around Heron Tower at the time," said Commander Karen Baxter.