Police said that the 17- and 18-year-olds had been threatening and demanding money from a group of four younger teenagers travelling on a train when the 50-year-old Munich businessman intervened.

He called police on his mobile phone and offered to leave the train with the children to make sure they were safe.

As they left the train at the Solln train station, the older teenagers followed and set upon the man.

Public prosecutor Larent Lafleur said in a press conference on Sunday the attack was an act of revenge and was morally at the lowest level.

The accused teenagers deliberately kicked the man in the fact, he said.

The man was left for dead, and the emergency services were called to the scene but doctors were unable to save the man’s life and he died onSaturday evening in the hospital from severe head injuries.

The teenagers are both said to be without work or training positions, and are both said to have criminal records for a number of offences.

They had run off after the attack but were arrested shortly afterwards.

Bavarian state interior minister Joachim Herrmann said, “I am appalled at this latest case of senseless and brutal violence.” He said it was a, “shocking example of the worrying increase in youth crime.”

Several brutal attacks have taken place in the past on Munich’s transportation system, including one which provoked a national debate on toughening the fight against juvenile delinquency.

The case in 2007 involved a 76-year-old pensioner who was beaten up by some youths at a Munich metro station after he asked them to put out their cigarettes. His two attackers were sentenced to long prison terms in July 2008.