Two men are dead after a horror crash involving multiple vehicles on Sydney Harbour Bridge.

NSW Police said four cars and a bus collided in the southbound lane about 1.40pm on Thursday, bringing traffic to a standstill.

Emergency services were called to the iconic bridge following reports of a multivehicle crash.

One man died tragically at the scene. A second was said to be in a critical condition, before dying at the scene later on Thursday.

Neither man has yet been formally identified.

NSW Ambulance paramedics also assessed several other patients. Acting Superintendent Clayton McDonald said later that all other drivers had been taken to hospital.

The bus driver was uninjured, and there were no passengers on his vehicle at the time.

“A crime scene has been established and officers attached to the crash investigations unit are investigating the incident,” NSW Police said.

They said a motorcyclist who allegedly rode through the crime scene had been arrested. He was taken to Day Street Police Station and was assisting with inquiries.

Transport for NSW coordinator-general Howard Collins said the bridge remained closed north and southbound.

“There are diversions in place, there is a significant traffic congestion in the area,” he said.

He said delays were likely to be protracted, even once the bridge reopened.

“There will be significant traffic to clear for the afternoon peak so advice for people, if you are travelling into the city, particularly those people who are considering travelling by road to the concert tonight at Moorpark, please think about alternative routes because the roads may well be closed or still congested,” Collins said.

Motorists were advised to check Live Traffic for the latest updates.

Police have asked anyone with dashcam or information to contact them or Crime Stoppers on 1800 333 000.