POTSDAM, Germany (Reuters) - Prosecutors opened their case on Tuesday against a 33-year-old man accused of killing two children, one of them a 4-year-old migrant boy kidnapped from Berlin's main migrant registration center - two murders that outraged the German public last year.

The handcuffed defendant, named only as Silvio S., confessed last year to killing both Mohamed Januzi, a Bosnian boy whose family was seeking asylum in Germany, and six-year-old Elias from Potsdam near Berlin, according to German prosecutors.

The defendant arrived in court with a paper file hiding his face, kicking off a trial taking place under strict security conditions. He faces a maximum sentence of life in prison.

Aldiana Januzi, the Bosnian boy's 26-year-old mother, testified that she had repeatedly warned her son to beware of adults with bad intentions, German media have reported.

"I explained that someone could try to trick him and hurt him," she told the court.

Januzi said she frantically searched the apartment complex for the boy after he went missing before calling police.

"They didn't take me seriously at first," local broadcaster Rundfunk Berlin-Brandenburg quoted her as saying. "When he didn't show up, they got more serious."

The defendant was arrested last October after his mother told police her son had informed her that he had been involved in Mohamed's abduction. The child's body was later found packed in cat litter in the boot of his car.

The Bosnian boy disappeared on Oct. 1 while waiting outside Berlin's central registration office for migrants with his mother and two siblings.

German prosecutors said the suspect had admitted to taking Mohamed to his flat and sexually abusing him. He strangled the child the following morning when he wouldn't stop crying.

The man has also confessed to having abducted and killed Elias, prosecutors said. German authorities have not disclosed Elias' surname.

(Reporting by Andrea Shalal; Editing by Gareth Jones)