Three Chinese were critically injured and a Croatian coach was taken to the hospital following a collision between a bus from the athletes' village and a van on the way to the Olympic rowing park.
The accident happened near the Shunyi Olympic Rowing-Canoeing Park.
Australian rowing team doctor Greg Lovell, a passenger on the bus, said three of the four people traveling in the van were critically injured in the "major" collision. Lovell was treated for a minor scrape on his knee.
"The bus collided with the van and in turn moved off the road and hit a guard rail," said Lovell, who was listening to music when he was jolted by the accident.
"We were able to get one of the passengers from the back of the van out onto the road, but the others were certainly very seriously injured."
The International Olympic Committee said two Croatian athletes and a coach were also on the bus.
Croatian coach Dragutin Milinkovic was injured and taken to a hospital, said Gordana Gacesa, a spokeswoman for the Croatian Olympic Committee. She said Milinkovic was struck in the head and had cuts and bruises.
"He was in shock but OK," Gacesa said.
Croatian rowers Mario Vekic and Ante Kusurin, a double sculls team, were not injured and finished fourth in their semifinal.
Jason Churchill and Marcus Free, relatives of 1996 bronze medalist Duncan Free, were in a taxi behind the van.
"It wasn't pretty," said Marcus Free, Duncan's brother. "There was one guy convulsing. We could not get him out, because there were no seats in the back of the van."
He said two of the passengers were already unconscious when they got to them.
The bus left the athletes' village around 1:40 p.m. local time, and the ride takes about an hour to the venue in Beixiaoying Town.
Dump trucks and nearly two dozen workers cleared glass and metal debris on the sidewalk near the accident.
Australian rower Matt Ryan said the bus he was on passed the accident scene near Dragon Villas, a residential compound in the suburbs of Beijing.
"It was a shock for us," he said. "We don't know much about it."