A teenager was dramatically arrested after scaling a 20-story crane in central Adelaide, following an intense negotiation that lasted over eight hours.

Authorities received reports around 8 p.m. yesterday that the young man had climbed to the edge of a crane at a construction site located at the intersection of King William and Gilbert streets.

The spectacle quickly attracted a crowd of onlookers, anxious for the teenager’s safety.

“At one point, he moved from the inside to the outside of the crane and looked down,” witness Mikaela Mudie shared with 9News.

“That was when we became really worried about his safety. We watched as he continued to climb higher and higher.”

Police officers called in the Special Tasks and Rescue Group and specialist negotiators to persuade the man to safely climb back down.

They tried to reach him from multiple angles, with police climbing up themselves and firefighters flying in from above.

About 4.30am today, the man finally climbed down.

The 19-year-old from Hackham West was arrested and taken to hospital for assessment.

Police expect to interview him and charge him with trespass and obstructing a public place. 

The crane itself will now need to be checked and calibrated, but it is unlikely to cause any delays to the building project.