The discovery of an unattended bag forced parts of Perth Airport to be cordoned off, throwing travel plans into turmoil and sparking a police investigation.

Passengers and staff were blocked from the terminal one check-in area this afternoon as federal and state police responded to the incident.

It’s understood a man entered the terminal making threats before dropping the bag and leaving.

Police are investigating after the discovery of an unattended bag forced parts of Perth Airport to be cordoned off, throwing travel plans into turmoil. (9News)

It’s also understood a man has been arrested in the city and is being questioned by police.

There were cheers as the terminal reopened just before 6.30pm (8.30pm AEST).

A Perth Airport spokesperson said an exclusion zone was established on the Terminal 1 forecourt as a precaution, forcing the closure of the terminal’s check‑in areas and short-term car park.

“The investigation has now concluded, and the impacted areas are beginning to reopen,” the spokesperson said.

“Perth Airport is working closely with airline partners and relevant agencies to return operations to normal as quickly as possible.

“Some delays are expected. Passengers are advised to check directly with their airline for the latest flight information.”

The Australian Federal Police said it was responding to the incident along with state police and the airport itself.

Members of the public were asked to avoid the area.

One passenger said the airport was obviously trying to keep people safe in the face of something serious but criticised its communication.

Another was concerned about what it would mean for her travel plans.

“[We’re] just wondering how long we’re going to have to wait and if we’re going to actually make our flights home tonight,” she told 9News.

“And what’s going to happen for the connecting flights. And it just sort of snowballs on from that really.”

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