An asylum seeker has been detained after he allegedly launched a string of sex attacks on 18 women during a single two-hour train journey.

The 23-year-old migrant is suspected of groping the women on a suburban S-Bahn German train line between Munich and Herrsching on January 29.

The Eritrean-born attacker is said to have gone from one carriage to the next touching female passengers.

The sickening spree was exposed after two victims reported their ordeal to the police.

Police identified the attacker after studying CCTV footage - when they found that a total of 18 women had been assaulted (file photo)

One 18-year-old victim told police he stroked her face and tried to force his hand between her thighs before she fled.

Another woman, aged 20, also told how he touched her legs until she ran to another compartment.

But when police viewed security CCTV from the train they realised that there had been 16 other sex attack victims on the train who had not come forward.

The 23-year-old migrant is suspected of groping the women on a suburban S-Bahn German train line between Munich and Herrsching

The attacker was on the train with an accomplice - who had watched him grope the victims but was not directly involved in the sexual assaults.

Police traced the man to a migrant camp in Starnberg, near Munich, where they found clothes resembling those worn by the man caught on CCTV.

The attacker, who was not named in reports, was arrested and held in custody, but police were not able to trace the second man, Focus Local reports.

The police investigation also identified the 23-year-old as a suspect in another attack in the city area of ​​Munich

Police have appealed for witnesses and the remaining 16 victims to come forward.

The police investigation also identified the 23-year-old as a suspect in another attack in the city area of ​​Munich on January 22, in which a man suddenly grabbed a woman's legs.

The clothes worn by the suspect in the Munich attack matched those worn by the attacker on the train.

Sabine Stein, Head of the Investigation Service of the Federal Police Inspection in Munich said: 'The rapid success of the investigation is due, among other things, to the good cooperation and the rapid exchange of information with the PI Herrsching and the K15 of the police department Munich, and the women who came forward.'