BERLIN — Berlin authorities said they arrested six people on Sunday over an alleged plot to carry out a "violent crime" at the German capital’s half marathon which attracted 36,000 runners.

A report by daily newspaper Die Welt which police did not confirm said that the main suspect in the probe planned to attack race participants and onlookers with two "specially sharpened knives".

It added that the suspect had links to Tunisian asylum-seeker Anis Amri, who carried out a deadly truck attack on a Christmas market in the German capital in December 2016.

A source close to the investigation, however, said that the Welt report that a knife attack was being planned "went too far".

Prosecutors and police said in a statement they had received "isolated indications that those arrested (Sunday) between the ages of 18 and 21 may have been involved in planning a violent crime in connection with this event".   

The security authorities said that on that basis and after a deadly van rampage on Saturday in the western German city of Münster, they decided to swoop on the suspects.

"Due to the still ongoing investigation, no further information can be provided at this time," they said.

The statement spoke only of "several" arrests but a tweet by the police mentioned six arrests carried out by special forces.

A police spokesman told AFP later that they had confiscated "data storage devices, mobile phones, two vehicles and a small knife" during the raids. He confirmed that the suspects arrested "might have planned something during the half marathon" but "we don’t know what exactly".

The half marathon went off early Sunday without incident, drawing large crowds amid warm summer sunshine.

Berlin’s top security official, Andreas Geisel, had said that the city of 3.5 million people would need to review its precautions for the half marathon following the Münster attack in which two people were killed.

Some 630 police officers were deployed to guard the race. — AFP