APELDOORN, Netherlands (AFP) - A car slammed into Dutch festival goers Thursday, killing four and injuring 13, as Queen Beatrix looked on in horror in what officials believe was an attack on the royal family. Investigators said they arrested the seriously injured driver of the black car which rammed into the foot of a monument after mowing down people gathered for the annual Queens Day royal procession in the central city of Apeldoorn. The driver is suspected of trying to carry out an attack on the royal family, prosecution service spokeswoman Ellen Prummel told AFP. The man, who is in hospital, also faces murder charges. Justice official Ludo Goossens told journalists in Apeldoorn that the 38-year-old driver probably acted deliberately. From initial contact with the man, before he was freed from the car, we have reason to believe that this was a deliberate act. There are no indications of terrorist links, Goossens added, nor were there signs of explosives. Investigators searched the Dutchmans house to try and establish a motive for his actions. Apeldoorn mayor Fred de Graaf said four people were killed and 13 injured - five of them seriously. The injured were taken to six hospitals in the region, where they are being treated. They include two members of the police. Television footage showed members of the royal family, in an open top bus, clutch their hands to their mouths in shock as the car sped through barricades right before their eyes and rammed into the foot of a monument, leaving injured people and battered bicycles in its wake. Beatrix, Crown Prince Willem Alexander and his wife Maxima were among the royals taking part in the procession. The car appears on television footage to have already been heavily damaged before slamming into the monument. Paramedics were seen rushing to assist people lying injured on the ground among pools of blood and scattered shoes, as anguished cries filled the air and bystanders started running around in obvious shock. A policeman on a bicycle managed to get out of the way of the car, identified as a small Suzuki Swift, just seconds before it hit the pillar. Many people narrowly escaped death, one witness told broadcaster NOS. I was standing near the monument and the car was coming straight at me. My legs are still shaking. Hundreds of thousands of Dutch took to the streets in several cities Thursday morning in orange wigs and bizarre hats to celebrate Queens Day, a national holiday and annual festival to mark the birthday of Beatrixs mother Juliana, who would have been 100 this year. But the festivities soon turned to mourning with official celebrations cancelled in many areas as the national flag was flown at half mast at the royal palace at Apeldoorn and government buildings. The streets of Apeldoorn, where crowds thronged earlier, had all but emptied a few hours after the incident and the festive decorations were taken down. Beatrix expressed shock at the events, telling the nation in a televised address she was speechless that something so terrible could have happened. Prime Minister Jan Peter Balkenende also expressed deep shock and described it as a sad day. European Commission President Jose Barroso sent a message of condolences, saying he was deeply shocked at the terrible news of the casualties at the Queens Day festivities in the Netherlands.