Eleven people were initially reported to have died in the accident, but Croatian police later stated that one more person died in hospital as a result of his injuries.
A total of 43 people traveled in the bus. The most likely cause of the accident is that the bus driver fell asleep at the wheel, reports a Croatian state television channel.
The accident is said to have occurred at 05:40 in the morning, five miles north of the Croatian capital Zagreb. The motorway stretch is heavily trafficked during the tourist season.
The bus was filled with pilgrims on their way to the village of Medjugorje, a Catholic shrine in southern Bosnia and Herzegovina, Polish Prime Minister Mateusz Morawiecki said.
The bus is said to have started near Poland’s capital, Warsaw