(CNN) About 2,200 nurses at the University of Chicago Medical Center went on strike Friday morning, causing the hospital to curtail services and transfer some patients to other hospitals.

The nurses' union said it began a 24-hour strike at 7 a.m. ET Friday. Nurses and their supporters wearing red shirts gathered in front of the hospital, chanting and carrying signs.

But the striking nurses won't be able to return to work until Wednesday morning, the hospital said, because it had to offer a five-day work guarantee to replacement nurses.

The 618-bed academic medical center is one of the most prestigious hospitals in Chicago. Its importance grew in 2018 when it became a Level 1 trauma center following a years-long effort by Southside residents and activists to reduce the travel time for victims of violence.

NURSES STRIKE: More than two thousand nurses hit the picket lines Friday morning at the University of Chicago Medical Center. https://t.co/poFvOV4mEo pic.twitter.com/FJ1heP3tvR — ABC 7 Chicago (@ABC7Chicago) September 20, 2019

Unionized registered nurses at the hospital voted August 30 to go on strike. The union says some of the main issues are chronic short staffing, nurses being required to work overtime every week, and nurses being required to work outside their areas of expertise, which puts patients at risk, according to a news release on the union website.

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