(Updates outage figures)

HOUSTON, Aug 24 (Reuters) - More than 310,000 customers in three Midwestern states were without power late Friday after a severe storm on Thursday and it will take several days to complete repairs, utility companies said on Friday.

A severe line of thunderstorms with winds of 50 to 70 mph and lightning uprooted trees and snapped power poles on Thursday afternoon across Chicago and northern Illinois, Indiana and southern Michigan.

Affected were customers of utility units of Exelon Corp <EXC.N>, CMS Corp <CMS.N> and American Electric Power Co <AEP.N>. Some customers of Chicago-based Commonwealth Edison were also without power.

"This series of storms has been the worst we've seen in the past 10 years," ComEd CEO Frank Clark said in a release.

Flooding in some areas and the large number of down tree limbs will take days to clear, utility officials said.

ComEd was working to restore electricity to 181,000 customers left without power, a spokesman said late Friday.

The storm affected service to about 600,000 customers as it moved eastward, the spokesman said. Hardest-hit areas were on the North Side of Chicago and the northern suburbs.

About 105,000 customers in Mt. Prospect and other adjacent suburbs on the northside had no electricity late Friday along with 46,000 customers in the city of Chicago.

Jackson, Michigan-based CMS said about 100,000 customers remained without power across that state late Friday, down from a peak of about 240,000 since storms moved into Michigan on Wednesday.

About 29,000 customers of AEP utilities in Michigan and Indiana were also without power late Friday, down from a peak of 34,600, the company said on its Web site.

Utilities have called in crews from as far away as Texas and Kentucky to speed the clearing of tree limbs and repairs to power lines.

Electricity will be restored for a majority of customers by Monday, but the utilities warned that some customers may be without power until Tuesday, given the significant damage to electrical equipment and flooding which limits access by repair crews.

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