A wall of the Copthorne Christchurch, which is undergoing demolition, collapsed unexpectedly on Saturday.

A wall of the Copthorne Christchurch, which is undergoing demolition, collapsed unexpectedly on Saturday.

The Copthorne Christchurch as it looked on Friday morning while under controlled demolition.

A wall of the Copthorne Christchurch, which is undergoing demolition, collapsed unexpectedly on Saturday.

A wall of the Copthorne Christchurch, which is undergoing demolition, collapsed unexpectedly on Saturday.

A wall of the Copthorne Christchurch, which is undergoing demolition, collapsed unexpectedly on Saturday.

The former Copthorne Hotel "pancaked" during demolition work in the Christchurch CBD today.

Reports on Twitter said demolition workers ran for their lives after the building collapsed in Victoria Square.

Witness Craig Crew said a nibbler was working about six floors up, eating at a vertical beam at the eastern side of the building when a front verandah fell down.

"Next thing the entire building pancaked on itself," Crew said.

The collapse happened about 1.25pm, with people in the area mistaking the impact of the fall for an earthquake.

Jill Ineson was on a bike ride around central Christchurch when she saw the building collapse.

"It happened so quickly. It fell down like a pack of cards," she said.

"The middle of the building just imploded like a pack of cards. It was exactly like the Twin Towers."

"There was a massive massive cloud of dust and you have a sick feeling in your gut and you hope that none of the guys were stuck in that. It was a bit like a massive earthquake. The ground shook. All these ghastly feelings rush into your head.

"After the dust settled, the contractors started to creep around looking pretty ashen-faced."

There were no injuries.

The Copthorne Hotel building was under demolition by Southern Demolition and Salvage. The building's owner had refused offers of help with the demolition from the Canterbury Earthquake Recovery Authority.

Southern Demolition and Salvage owner Alan Edge would not comment on what caused the collapse or what contractors were doing at the time of the collapse. He said he was waiting on a report from CERA engineers who were investigating.

Police are monitoring the site.