ARMED police stormed an office building in Sydney’s CBD yesterday after claims a gunman was on the premises.

Oxford Street and Wentworth Street were closed off just after 4.30pm and workers were evacuated while police carried out a search of the building.

It is understood the operation was sparked when someone inside the building sent a message to a friend saying there was a gunman in the building, The Daily Telegraph reported.

Nine News reported that a threat was made against the Fair Work Commission, however this has not been confirmed.

Police searched the building for several hours but were unable to find anything.

Just before 7.30pm NSW Police tweeted the operation at Darlinghurst was now over.

#Darlinghurst office search completed, nil found, invest. now underway, thanks everyone for patience with disruption to traffic etc. — NSW Police (@nswpolice) November 18, 2015

A NSW Police spokeswoman told news.com.au that ambulances and firefighters who had been on stand-by had been stood down just before 7pm.

She also said a considerable number of police resources had also been stood down.

Earlier, a local resident who lives nearby told news.com.au said the area was “swarming with police” and the atmosphere was tense.

She said she left the area but was worried about a friend who works in the building, which houses the Australian Federal Police lawyers, the Department of Education and Training, the NSW Workers Compensation Commission and the Public Defenders Chambers.