APELDOORN: A car slammed into Dutch festival goers Thursday, killing four and injuring 13 as Queen Beatrix looked on in horror at what the driver admitted was an attack on the royal family.

Officials 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 Queen's Day royal procession in the central city of Apeldoorn.

"At the moment of first contact with police on the ground, the man indicated that his action was aimed against the royal family," justice official Ludo Goossens told journalists in Apeldoorn.

The 38-year-old Dutchman was taken into custody, but was hospitalised with critical injuries.

"His condition is life threatening," Goossens said.

If he survives, he will be charged with attempting an attack on the royal family, and murder.

"There are no indications of terrorist links," Goossens said, nor are there signs of explosives or of a broader conspiracy.

Investigators searched the man's house to try and establish a motive. He had no record of mental illness or previous crimes.

Apeldoorn mayor Fred de Graaf said two men and two women were killed and 13 people injured -- five of them seriously. Three among the injured were children aged nine, 15 and 16.

"The injured were taken to six hospitals in the region, where they are being treated," the mayor said.

They include two members of the police.

Some suffered head trauma, but most had bruises and broken bones.

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 Queen's Day, a national holiday and annual festival to mark the birthday of Beatrix's 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 Queen's Day festivities in the Netherlands".

A total of 250 investigators were put on the case.