The terrorists stormed the police training centre's hostel on Saryab road at around 11:30 pm last night and started firing.
The attackers are currently inside and engaged in an intermittent exchange of fire with security forces.
"At least 20 people have been injured in the attack," a senior provincial official was quoted as saying by the Express Tribune.
"5-6 terrorists attacked police training centre hostel. Army and FC troops arrived on the site. Operation is underway," Inter-Services Public Relations (ISPR) said in a statement.
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Around 200 cadets are said to be present in the centre's hostel.
Balochistan's Home Minister Mir Sarfaraz Ahmed Bugti confirmed that terrorists have attacked the training centre.
"Our security forces have the capability to combat such attacks and threats," the home minister told reporters.
He added that that recently there were 700 cadets present, but a batch passed out a few days ago.
"As of now, I do not have a confirmed figure on cadets present inside the centre," he said.
The injured are in stable condition, said the police official.
Extra contingents of security forces have been deployed at the spot and a cordon has been established.
"Exchange of firing is going on but it is not clear how many hostages might have been taken by the attackers," a spokesman for the Baluchistan government said.
But he conceded that at a time at least around 200 trainees took training at the centre.
Militants have conducted attacks against security forces and national installations in Balochistan, which has been plagued by an insurgency and growing sectarian killings for more than a decade.