A 77-year-old man has allegedly been shot inside a Sydney CBD office building by a 79-year-old man wielding a semi-automatic pistol.

The alleged shooter was held down by a crowd of staff members as others rushed to stem the bleeding of the injured victim.

Police were called to 219 Castlereagh Street just before 1pm on Wednesday to reports of multiple people shot but later confirmed there was only one victim.

They found the victim with a gunshot wound to his groin, Detective Inspector David El-Badawi told reporters.

“He was treated by paramedics. He’s in a stable condition and was taken to St Vincent’s Hospital,” El-Badawi said.

“The offender is also in his mid-70s and he’s currently at Day Street [police station].”

El-Badawi said the five or six staff members who pinned down the alleged shooter were “very brave” and police had arrived on scene in three minutes.

The alleged shooter is not co-operating with police and investigators said it was too early to know if the attack was targeted.

“We don’t believe it’s connected to any outlaw motorcycle gangs or any gang warfare, anything like that,” El Badawi said.