Zeeshan Afzal, an officer from Punjab Highway Patrolling Police, told Xinhua that the accident happened at around 10.00 p.m. in Dhok Pathan area of Pindi Ghep region of Attock.
Local people, police and rescue teams rushed to the crash site and shifted the bodies and injured to hospitals in nearby cities.
At least 18 people died on the spot while four others succumbed to their injuries later.
A hospital official said that the death toll might further rise as seven injured were still in critical condition.
The doomed bus was carrying 82 passengers to a religious congregation in the provincial capital of Lahore from Attock.
The police official said the driver of the ill-fated bus had little experience and could not control the bus while making a sharp turn in high speed.
Shahbaz Sharif, Chief Minister of Punjab, expressed deep grief and sorrow over the loss of precious lives in the unfortunate accident.
According to official data released by the Pakistan Bureau of Statistics, around 9,000 road accidents are reported every year since 2011, killing more than 4,500 people on average annually.
Traffic police said that 90 percent of the accidents in Pakistan are caused merely by human error.