The tragedy came days after a devastating oil tanker explosion killed at least 205 people. Pakistan has a poor record of fatal traffic accidents due to badly maintained roads and vehicles, and reckless driving.
"A speeding passenger bus which was going to Rawalpindi from Karachi first hit a tractor trolley and then a tree, killing 11 people and wounding 36 others, 18 among them seriously, near Layyah town," a senior local administration official in Layyah, Wajid Ali Shah told AFP.
The oil tanker overturned on a main highway from Karachi to Lahore while carrying some 40,000 litres of fuel on June 25, about 270 kilometres from Layyah.
It exploded minutes later as crowds from a nearby village gathered to scavenge for fuel, despite warnings by the driver as well as motorway police to stay away.
Officials have launched an investigation into the accident.
The tragedy marked a grim start to Eid, the celebrations closing the Muslim holy month of Ramadan.