The crash near Dresden occurred shortly before 2 am when a Polish bus hit the rear of a Ukrainian bus and then broke through the median barrier, slamming into a Polish minibus travelling in the opposite direction, police said.
The larger vehicle partly crushed the minibus before rolling down a verge.
"In my 36 years of work as a professional firefighter, I've never witnessed an operation with such a large number of dead and injured," Dresden fire chief Andreas Ruempel told German public broadcaster MDR.
So far, three men and two women, all of them Polish citizens, have been identified among the dead, Ulbricht said.
The driver of the Polish bus was among the injured. Prosecutors have opened an investigation against him on suspicion of negligent homicide.
Of the 21 persons travelling on the Ukrainian bus, four received treatment for minor injuries.