ACT Policing Detective Inspector Matthew Reynolds said a man behaving erratically left a bundle of clothing in Civic Library on Wednesday and made several concerning comments to community members about bombs, who alerted police just before midday.

"The item ... was not causing us any concerns however the comments made by the person who left the item there and his behaviour led us to have some concerns in relation to that item," Detective Inspector Reynolds said.

The library and surrounding buildings were evacuated as a precaution while ACT Policing's bomb response team assessed the threat of the item.

A bomb-detection robot was also deployed to the library.

Detective Inspector Reynolds said the item was examined and found not to be of concern.

Police earlier said the man was known to them. Detective Inspector Reynolds said he could face charges including causing public alarm and public mischief for the alleged "serious offence".

"And that's to start with," he said.

"We need to understand what the motivation was, it is possible the person may have a mental health issue."

"Once we ascertain what's happened we'll be able to make further comment there."

The nearby Legislative Assembly was not evacuated as Detective Inspector Reynolds said the bomb team assessed it as being a safe distance while they determined the threat.

London Circuit between Northbourne Avenue and Akuna Street was closed for some time and Civic Square had been taped off while police and emergency services were on the scene.

Detective Inspector Reynolds said the scare was "frustrating" but said people shouldn't hesitate to contact police about bomb threats.

"ACT Policing takes all bomb threats seriously. We would ask people to contact police in the event of a bomb threat, provide as much detail to us as you can," he said.

"In this case members of the public were able us to provide descriptions of the person as well as clothing and we're also able to obtain CCTV."

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