Firefighters are searching for people feared trapped under collapsed scaffolding at an old shopping centre.
The structure came down just after 11:15 BST in Garrard Street, Reading, injuring at least three people.
Royal Berkshire Fire and Rescue Service is now using thermal imaging equipment to search the site, which used to be the Friars Walk shopping centre.
The three people taken to hospital – a site worker and a couple who had been passing – had minor injuries.
Tony Heselton, incident commander for South Central Ambulance Service, said the worker suffered a head injury while the woman had an ankle injury and her partner was in suspected shock.
Witness Louis Stead said he saw the corner of the building fall with an “almighty crash and lots of dust and debris”.
“It was horrifying, very shocking and very concerning,” he said.
Another witness, Gearoid Fallon, said: “The building itself is being torn down so I just assumed it was part of a demolition thing.
“A few of the work crew looked quite distraught.”
Thames Valley Police said it had temporarily closed the nearby NCP car park to the public, but there were “no concerns” about the building’s structure.
A spokeswoman for NCP said it would not charge any customers delayed by the incident.
At the scene: Matt Graveling, BBC South Today
People from local businesses are being told they cannot yet get their cars from the NCP car park, with police guarding the entrances and exits.
Some people with meetings and businesses within the cordon are being escorted by officers to their place of work.
Search and rescue dogs are being taken in and out of the site and the emergency services have widened the cordon to take in a number of roads.
Eyewitness Sabina Jeske said she had been chatting to a client when they heard a builder shouting at people to “run away”.
“People started running away and when everything collapsed one lady was caught by the scaffolding and cut her hand,” she added.
A builder, who was working on a scaffolding site across the road from the collapse, said there was “panic”.
“We heard a loud bang,” he added.
“It sounded unusual… so I stuck my head around the corner, and seen the aftermath of the scaffolding collapsing across the street.”
Garrard Street has been closed in both directions and the police cordon has been extended.
Greyfriars Road has also been closed and people are urged to avoid the area.
The derelict shopping centre is being demolished as part of a £500m housing and retail development at Station Hill in Reading.
The BBC reported in June property firm Lincoln MGT had snapped up the highly sought-after development site, becoming its third owner in 13 years.