Updated 05/20/12 - 12:44 p.m.
CHICAGO (CBS) -- Authorities are investigating whether the City Of Chicago's website is under a cyber attack, CBS 2 has learned.
The main portal to cityofchicago.org was not accessible for part of Sunday morning, but appeared to be functioning again by about 12:40 p.m.
"We are aware of the potential issue of the City of Chicago website and are working with the appropriate federal authorities to address the situation," Office Of Emergency Management and Communications spokesman Pete Scales said earlier Sunday.
A posting on the website cyberwarnews.info, explains the hacking group called Anonymous started an "an operation toward the Chicago police department and its websites."
The Chicago Police Department website, cityofchicago.org/police, was also down temporarily, but was also functioning again Sunday afternoon.
"We are actively engaged in actions against the Chicago Police Deptartment [sic], and encourage anyone to take up the cause," the group said. "We are in your harbor Chicago, and you will not forget us."
The group explains the attack is in response to the NATO Summit being held in Chicago. The summit officially opens this afternoon and runs through Monday.
Thousands of protesters have been filling the streets of Chicago all week to protest NATO's mission. The biggest rally yet is planned for Sunday afternoon from Grant Park to near McCormick Place, where leaders are gathered for the summit.
Hackers may also be attacking other NATO-related websites, but details were not available, sources to CBS 2 Chief Correspondent Jay Levine.
On Twitter, allies of the Occupy protest movement applauded the development:
A press liaison for Occupy Chicago praised the apparent hack in a tweet:
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