Police have charged a man after allegedly uncovering guns, drugs and explosives at an apartment on Grenfell Street in the heart of Adelaide's CBD — but say the incident was "not terror related".

Key points: Explosives were found during a search of a building in the heart of Adelaide

Explosives were found during a search of a building in the heart of Adelaide A witness said he saw a man detained before police shut the area

A witness said he saw a man detained before police shut the area Police have described the items as "commercial explosives"

The explosives were allegedly found about 12:30pm when officers were conducting a search at the property inside the Princes Apartments tower block as part of an ongoing investigation.

Members of the Bomb Response Unit examined items — described by police as "commercial explosives" — at the scene.

Chief Inspector Michael Walsh said the search of the apartment — inside the 12-storey tower block at 39 Grenfell Street — began about 9:30am after receiving information from the public.

"Police have located two firearms — a 22-calibre firearm and an air rifle. They have also located some drugs and drug paraphernalia and also located explosives," he said.

"As a result of that search and the safety elements, we have called in [the] bomb response [unit].

"The bomb squad has confirmed it is an explosive device. It is commercial explosives. At the moment they are ensuring the area is safe."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 44 seconds 44 s The bomb squad was called to Adelaide's CBD after explosives were found.

Police said they also located ammunition and have now removed the explosives, which are being taken away to be detonated.

A 40-year-old man who is known to police was arrested and charged "with a range of firearm, drugs and explosives offences", and was expected to face court on Thursday.

"Police will remain on scene until the area is deemed safe," Inspector Walsh said earlier.

Police have described the items as "commercial explosives". ( ABC News: Helen Frost )

He said the incident was "contained" to "one apartment at the front of the premises".

"We haven't exactly confirmed what the explosives were. But all explosives do have the potential to cause danger," he said.

"That's why we have had this response. It's for the safety of the occupants and the CBD business premises."

'No-one allowed in or out', cafe owner says

Police said there was "no risk to the public" and the decision was made not to evacuate surrounding apartments, the building or the area.

Grenfell Street remained open to traffic during the incident, but the surrounding footpaths were closed to pedestrians.

Loading

The Metropolitan Fire Service were among emergency services at the scene.

Neighbouring cafe owner Fadi Mawlawi, whose business is about 10 metres from the building where the explosives were located, was forced to close his doors when the area was closed to foot traffic.

He said police arrived on Wednesday morning and he witnessed them detaining a man several hours before the alarm was raised about explosives.

"They must have been searching the place where he lived," he said.

"They had him sitting on the ground floor for over an hour and then they started blocking off the area and said there was a risk of explosion here.

"It's pretty dangerous, no-one's allowed in and out.

"My background coming from Lebanon — I've lived there before, I'm used to it, but you've got to take it seriously."

Hairdresser Alexandra Salmeron, who works nearby, told the ABC she noticed a strong police presence in the area.

"It got a bit busy, a bit hectic, and then clients came in and they're like 'something's going on'," she said.

"We stayed put for the time being and waited.

"It is a bit scary — things like this happening in little old Adelaide, it's a bit of a shock.

"There's a lot of people in these buildings."

Surrounding office workers have witnessed the situation unfold. ( Supplied )

Neha Pathrella said she was walking down the street on her break when she saw the scene.

"I just hope everyone in the building is safe and has been evacuated," she said.