Protesters breach fence line near United Center on Day 1 of DNC; at least 4 arrested

CHICAGO -- Protesters breached an outer fence line on the north side of the United Center Monday evening on the first day of the Democratic National Convention.

Capitol police wearing gas masks could be seen in the area around 5 p.m.

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More police officers wearing riot gear later approached.

At least four people were taken into custody, as police formed a barricade to block the breached fencing, on the south end of Park 578 in the 1900-block of West Washington Street.

Chicago Police Superintendent Larry Snelling was in the area, overseeing the response.

The inner perimeter was not breached, and there was no threat to anyone inside, the DNC Public Safety Joint Information Center said in a statement.

"The National Special Security Event plan employs a multi-layered approach. Each NSSE plan is unique and includes multiple layers. One of these redundancies include the outer perimeter fence, which was erected to prevent entry into the inner perimeter," the statement said.

CPD remained on the scene, clearing the area just after 6 p.m.

This is a developing story. Check back for updates.