Four people have been taken to hospital after a concrete pump line blew out at a busy building site in the Melbourne CBD on Thursday.

Emergency services were called to the incident at a work site on Bourke Street just before 9am on Thursday.

It is believed the pump line of a concrete truck blew out and injured four workers, police said.

There were reports of paramedics wheeling injured people from a building site on Bourke Street, near the Spring Street intersection. The nature of their injuries and their age and conditions is not yet known.

Tram services along Bourke Street were halted after the accident. It is not yet known how long they will be out of action.

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Ambulance Victoria said paramedics treated four people at the scene.

Two men — one believed to be in his 30s and another in his 20s — were transported to the Royal Melbourne Hospital in stable conditions after being treated for upper body injuries.

The other two men, both believed to be aged in their 30s, were taken to The Alfred hospital in stable condition after being treated for upper body injuries.

Traffic was diverted from the work site as emergency services presence attended the scene.

WorkSafe has been notified and will investigate.

-more to come