About 20 supporters of the Catholic Workers, a social-justice, anti-war group calling for an end to NATO military involvement in the Middle East, showed up Monday at Prudential Plaza, worked their way to the second floor, where an attempt to reach Obama's campaign offices stalled when security guards shut down the elevators, the Chicago Tribune reported.
Eight protesters volunteered to be arrested after they read a statement and the rest left the building peacefully, police said.
"I was definitely expecting something like this this week, but I didn't expect it to start now, on Monday," Matt Archambault, who left his office to watch the events, told the Tribune. "It's a good test to see how things are going to go the rest of the week."
Other protests are expected to be conducted alongside larger rallies planned for Friday and Sunday. The NATO summit runs Sunday and Monday.
"This is what we said all summer when this was announced ... that there would be protests all over the city," said Joseph Iosbaker, an organizer for Occupy Chicago. "There's going to be something every day. A lot of things."
The eight demonstrators arrested Monday were charged with criminal trespass, a misdemeanor.
Chicago Mayor Rahm Emanuel said the demonstration showed how the city would handle demonstrations during the run-up to the NATO summit.
"I think police are doing their job. The other people are having their job to express themselves," Emanuel said. "As I've said before, we'll protect public safety as well as public speech. Both will be achieved."