Group of balaclava-clad men storm train, held by police

After boarding a train, the group was stopped at North Sydney station by more than 20 police officers.

Passengers not involved with the group have been evacuated, a spokeswoman for Transport for NSW said.

As of 4pm a NSW Police operation was still underway.

"The men are being taken off the train, but slowly," a NSW Police spokeswoman told AAP.

The train is due to begin moving shortly, a Transport for NSW spokeswoman added.