At least two policemen were killed in the attack, while at least 51 have been injured and were treated at Civil Hospital. Three were said to be in critical condition.
At least three of the attackers have also been killed.
The attack started at 9.30 p.m. on Monday as the attackers reportedly entered the complex through the front gate after shooting the guard, while others climbed the rear wall of the training centre.
About five or six militants entered the hostel of the police training centre, which houses nearly 200 cadets and opened fire on recruits.
The attackers are currently inside the building where an intermittent exchange of fire is ongoing with the security forces.
Three loud explosions, which occurred inside the college, were also heard.
Police and army personnel have surrounded the vicinity in large numbers as the operation is continuing inside the premises.
An emergency has been declared at hospitals around the provincial capital.
“I am myself monitoring the situation. Our forces have entered into the centre and soon these terrorists will be eliminated,” Geo Tv quoted Balochistan Chief Minister Sanaullah Zehri as saying.
The training college is situated on Sariab Road, which is one of the most sensitive areas of Quetta. Militants have been targeting security forces in the area for almost a decade.
The training college has come under attack in the past in 2008 and 2006, with attackers firing rockets into the college playground.
In August, a suicide bombing at a Quetta hospital killed 73 persons. The attack was later claimed by the Islamic State group and the Jamaat-ul-Ahrar.