A car travelling at high speed ploughed into a crowd waiting for Queen Beatrix of Holland, reportedly killing up to four people.
Witnesses said that the black Suzuki Swift appeared to deliberately target an open bus carrying Queen Beatrix and her family in the town of Apeldoorn during festivities to mark Queen's Day, a national holiday.
The car swerved across police railings, where crowds of people were waiting to see the queen pass, and slammed into the foot of a stone monument, where it came to a halt, its bonnet crumpled and scraped.
The royal bus was not hit, and no one in the queen's entourage was injured, officials said.
But television footage showed members of the royal family, riding in the open top bus, clutch their hands to their mouths in shock as the car sped through a barricade right before their eyes.
From television footage of the incident, the car appeared to have already been heavily damaged before crashing into the monument.
A policeman narrowly escaped injury when he jumped off his bicycle to avoid being hit.
Dutch television said that at least 14 people were mown down and injured, but police in Apeldoorn, about 55 miles from Amsterdam, declined to confirm the report.
Police removed a white male from the vehicle and put him into an ambulance. Investigators said a press conference would be held later today.
The Dutch ministry of royal affairs announced that the Apeldoorn celebrations were cut short after the incident.
"Her majesty the queen, on behalf of other members of the royal family, expresses her shock at the incident and conveys her condolences to the victims and their next of kin," said a statement.
Hundreds of thousands of Dutch took to the streets in several cities in orange wigs and bizarre hats to celebrate Queen's Day, an annual festival and national holiday for the Netherlands.