UPDATED: 5/19/2012, 9:10 p.m.

CHICAGO (CBS) – Protesters clashed several times on Saturday with police in the Loop, shutting down streets and creating traffic jams during a meandering march through the city over several hours.

Demonstrators had shut down South Michigan Avenue and were chanting, standing and sitting in the street near Roosevelt University at 8:30 p.m. before moving on.

They then moved down Michigan Avenue and had a standoff with police on horseback near the Art Institute — where Mrs. Obama is scheduled to hold a NATO reception on Sunday. Just past 9 p.m., they turned up Randolph, heading west. There was a brief skirmish outside the Walgreen’s at the corner of State and Randolph. Eventually, the group ended back at LaSalle and Jackson, where the march started in the mid-afternoon.

Some demonstrators got aggressive with police and surrounded police vehicles. In one incident, a CBS 2 photographer saw a surrounded police van lurch forward, which pushed a protester onto the ground near Willis Tower at about 9:45 p.m. The demonstrator was transported to Northwestern Memorial Hospital, but details weren’t available.

Around 7:30 p.m., a group of several hundred were seen scuffling with police outside the CBS 2 Broadcast Center at Dearborn and Washington. Several police in riot helmets let the demonstrators pass after the brief encounter.

Chicago Police Supt. Garry McCarthy said police anticipated the tactics of the protesters and were responding with restraint.

“It’s like herding cats, but the cops are doing an amazing job,” he told CBS 2′s Mike Puccinelli.

A few minutes after the Washington-Dearborn confrontation, police clashed again with protesters at Balbo and State and police were seen arresting demonstrators. One protester told CBS 2 he was trying to challenge the police line.

Officers on horseback were seen trying to control the crowd.

WBBM Newsradio’s Mike Krauser reported one protester was struck in the face by an officer and required medical treatment. There have been reports of several arrests, but the exact number is not known.

The protesters then moved east down Washington and then headed south on State Street. They eventually ended up near Grant Park.

Earlier, protesters clashed with police on Saturday in the South Loop.

The group was seen scuffling with officers who had blocked their march at 18th street just west of Michigan Avenue.

At least one protester was arrested after shoving with officers around 5:20 p.m.

The group had started its march around 3:30 p.m. at LaSalle and Jackson to protest the arrest of the so-called NATO 3, three visiting demonstrators who were charged with plotting to attack key targets with Molotov cocktails.

They had marched in the Loop to Daley Plaza at the start of their protest. They then began moving east on Washington Street to Michigan Avenue, where they proceeded south to 18th.

Protesters sit in Grant Park following a five hour march through the streets of downtown Chicago. (Credit: Rick Kramer/CBS Chicago)

After that altercation the group, which was estimated at a few hundred people, began walking back north on State Street. They then were blocked by police at State and Harrison. The streets around that area were shut down briefly.

After a brief standoff, the group then headed west on Harrison.