Cyclist Dylan Groenewegen apologised on Thursday after causing a crash in the opening stage of the Tour of Poland which left fellow Dutchman Fabio Jakobsen in a coma.

"I think what happened yesterday is terrible. I can't find the words to describe how sorry I am for Fabio and the others who fell or were affected," he wrote on Twitter, a day after the crash.

Doctors said Jakobsen, of the Deceuninck-Quick Step team, was in a "serious but stable condition".

He underwent a five-hour operation to the head.

"A scan has been carried out and the brain does not appear to have been affected," Pawel Gruenpeter, deputy head of the Sosnowiec hospital where Jakobsen is being treated, told Polish media.

"The main injuries are to the face. Fortunately the eyes have not been affected. The condition is serious but stable," Gruenpeter said.

The doctor also said medical staff would try to wake Jakobsen from his coma on Thursday but a hospital spokesman later said that this process would be "gradual" and not be complete until Friday.

Jakobsen, 23, was thrown into and over a barrier at 80 kilometres (50 miles) an hour as he raced elbow-to-elbow with Groenewegen of Jumbo-Visma in a tight sprint to the line in Katowice in southern Poland.

Groenewegen veered suddenly to the right, squeezing his rival into the security wall. Jakobsen somersaulted over the barriers before colliding with a race official.

Groenewegen went on to win the stage but was later disqualified with Jakobsen declared the winner. 

Medics were on hand following the incident which sparked multiple cyclists to crash

Organiser Czeslaw Lang said in a statement that he was "somewhat relieved" after speaking to doctors.

"After seeing the crash, we feared the worst, but now we know that the situation is serious but stabilised," Lang said.

Lang added that the race official also injured in the accident had "regained consciousness and is now in a stable condition".

Three other cyclists, Damien Touze, Marc Sarreau and Eduard Prades, are also still in hospital.

The incident came a year to the day after the death of 22-year-old Belgian sprinter Bjorg Lambrecht, who died after falling and hitting a concrete structure on the 2019 Tour of Poland.

Patrick Lefevere, general manager of the Deceuninck-Quick Step team, has called the incident a "criminal act" and said on Thursday that he would file a complaint to Polish police.

"We won't let this drop," he was quoted by Belga news agency as saying.

"It was a very dirty move from Groenewegen... I have watched the sprint dozens of times and I cannot fathom why Groenewegen did that."

Polish prosecutors are examining whether to open a case.

Governing body UCI earlier said it "strongly condemns the dangerous behaviour of Dylan Groenewegen" and has referred the incident to a disciplinary panel.

The drama came at the end of the first stage, raced over 198km from Chorzow to Katowice in southern Poland.

Jakobsen is considered a rising star of the sprint in the peloton who made his name in 2019 with two stage wins on the Vuelta a Espana, one of the sport's three Grand Tours.

Having turned professional in 2018 with Patrick Lefevere's Quick-Step team, Jakobsen donned the Dutch champion's jersey in June last year. 

In last year's Tour of Poland he was third on the opening stage.

Groenewegen, 27, is a four-time stage winner on the Tour de France, including the final stage in 2017 on the Champs Elysees in Paris.