CHICAGO (Reuters) - Gusts of up to 70 mph (113 kph) in the Windy City on Friday caused the closure of the second-tallest U.S. building's observation deck, power outages, flight cancellations and highrise evacuations.

The Willis Tower Skydeck, a popular tourist attraction at the 1,451-foot-tall (442-m-tall) Willis Tower, said it closed due to the weather.

The Chicago Fire Department said it was evacuating tenants from three downtown highrises due to blowing debris such as broken glass from nearby buildings caused by the high winds.

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The third-largest U.S. city's aviation department reported delays at O'Hare International Airport of up to 50 minutes and 160 canceled flights.

The National Weather Service issued a high wind advisory for the area in effect until 6 p.m. CST. The agency reported fallen trees and power lines, road accidents and the collapse of a building under construction.

On city streets, pedestrians struggled to stay upright and retrieve lost hats and papers.

Flying debris stopped service on one branch of the Chicago "L" rapid transit rail and caused major delays on another, according to the Chicago Transit Authority web site.

A suburban commuter train line was delayed when a large box containing salt used to clear ice from the platform fell onto the tracks, said Metra spokeswoman Meg Reile.

About 92,000 customers were without power in Northern Illinois on Friday afternoon, while another 91,700 who had lost power had service restored, said ComEd spokesman Alejandro Bodipo-Memba.

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(Reporting by Mary Wisniewski and Fiona Ortiz in Chicago; Editing by Cynthia Osterman)