Two of the 13 casualties are confirmed to be suffering from life-threatening injuries.
And, according to The Mirror, another six to eight ‘walking wounded’ passengers were left with scrapes and bruises following the crash.
In a statement, Sussex Police have confirmed that the accident is being treated as a ‘major incident’.
It reads: "Two people have sustained life-threatening injuries and three have been seriously injured in a crash between two buses in central Brighton.
"Emergency services were called to Queen's Road, near the Clock Tower, at 9.22am on Monday.
“It has been declared a major incident by the ambulance service and police have advised drivers to avoid the city centre for the next few hours.”
The statement continues: “In addition to those seriously injured, six to eight people are understood to be 'walking wounded' with minor injuries and a walk in centre for anyone injured has been set up at the top of Queen's Road near the railway station.
"West Street is being used as a holding point for ambulances attending the incident. A triage centre has been set up on North Street."
Meanwhile a spokesperson for Brighton and Hove Buses has confirmed that a full investigation will be launched into the shock collision.
They said: "Our first concerns are for the safety and well-being of our staff and passengers on the bus. We have also set a full investigation in motion.
"There will be severe disruption to services this morning and diversions are in place with eastbound services diverting via Queens Road and North Road and westbound services diverting via the A259 and Holland Road.
"Any services due to terminate at Churchill Square are now terminating and re-starting at the Steine, including service 25."
We will bring you more on this story as it updates.