The Gutenberg school is well-known in Erfurt
Masked and dressed in black, the gunman walked through classrooms killing 14 teachers, two schoolgirls and one of the first policemen on the scene before taking his own life.
Pupils of the Gutenberg School spent four hours trapped inside before police could declare the building safe.
Chancellor Gerhard Schroeder described the massacre in the quiet provincial city as "beyond the powers of the imagination".
As four other victims were being treated in hospital, people gathered in the city for a church service in the evening.
It is the worst school massacre in Europe since the 1996 shooting in Dunblane, Scotland, when a deranged gunman killed 16 children, a teacher and himself.
BBC Berlin correspondent Rob Broomby says the incident is also the worst of its kind in Germany's post-war history.
The German authorities have not given the name of the Erfurt killer but they said he was a 19-year-old who had been expelled from the school several months earlier and told he could not sit his university entrance exam.
Exams
"We were sitting in class doing our work and we heard a shooting sound," said eyewitness Filip Niemann.
Major school shootings:
When the killer turned up on Friday morning with a pump-action shotgun and a pistol, his former classmates were sitting exams.
"We joked about it and the teacher smiled," said Niemann.
"The teacher let us go out and see what was happening and when we left the classroom, three to four metres in front of us, there was a masked person in black holding his gun at his shoulder."
Niemann saw a teacher being shot and fled with other pupils as the killer stalked the classrooms, searching for more teachers.
Fifteen of those killed died in the first few minutes of the shooting, which began shortly before 1100 local time (0900 GMT).
After a caretaker alerted the police, two officers appeared at the school around noon, only to find dead bodies as they entered.
As they approached, the gunman shot dead one of them, a 42-year-old father who had been planning to celebrate his daughter's birthday that day.
The police then sealed the area and brought in special forces as scores of children remained trapped inside.
A handwritten note could be seen attached to a classroom window saying simply "Help".
The killer took his own life, reportedly when commandos finally stormed the building.
"This student seems to have had such hatred because he was expelled and couldn't sit his exam that he was driven to this terrible deed," said Interior Minister Otto Schily.
By a bizarre coincidence, he noted, the German parliament had passed new legislation tightening gun controls on Friday.