The crash took place near Dhok Pathan village, some 115 kilometres (70 miles) southwest of the capital Islamabad late Wednesday.
"At least 26 people have died and 46 others were injured when a passenger bus lost control at a slope and veered off into a deep ravine," local police official Fazal Abbas told AFP.
The bus was carrying members of Tableeghi Jamaat -- a Sunni evangelical group -- from the northwestern town of Kohat to the eastern city of Raiwind, near Lahore where their annual gathering was taking place, he added.
Abbas said that most of the injured were discharged from hospital after receiving medical treatment, but at least 10 seriously wounded passengers were sent to Rawalpindi, a garrison city adjoining the capital Islamabad.
Officials added that the bus was not driving on its usual route when it crashed. Dense fog and traffic on the usual route had forced the bus to change its route and ply through hilly stretches.
A local government official confirmed the accident and toll.
Pakistan has one of the world's worst records for fatal traffic accidents, many of them blamed on poor roads, badly maintained vehicles and reckless driving.