The 23-year-old crashed heavily into the barriers while battling for stage victory with countryman Dylan Groenewegen.
Jakobsen was resuscitated at the scene in Katowice, southern Poland, and airlifted to a hospital in what was described as "serious" condition, Poland’s PAP news agency reported.
Several other cyclists and a race official suffered less serious injuries in the pile-up, according to the Polish state news agency.
Groenewegen, meanwhile, was disqualified and excluded from the race for causing the incident, public broadcaster Polish Radio’s IAR news agency reported.
Photo: PAP/Andrzej Grygiel
Poland’s largest cycling race started on Wednesday amid the coronavirus pandemic, attracting more than 150 riders from various countries.
The first leg of the annual event saw them cover a 195.8 km route starting in the city of Chorzów and ending in Katowice in Poland’s southern Silesia region.
Last year's Tour de Pologne was marred by the death of Belgian cyclist Bjorg Lambrecht who sustained serious injuries in an accident.
The 22-year-old hit a concrete culvert by the road during the third stage of the race from Chorzów to Zabrze in the south of Poland on August 5, 2019.
Source: TVP Sport, PAP, IAR, uci.org