CHICAGO– On the first day of the Democratic National Convention, protesters broke through an outer fence line on the north side of the United Center on Monday evening.

At around 5 p.m., police officers wearing gas masks were seen in the area

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Later, more police officers in riot gear approached.

At least four people were arrested when police set up a barricade to block the breached fence at the south end of Park 578 in the 1900 block of West Washington Street.

Chicago Police Superintendent Larry Snelling was on site and oversaw the operations.

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

“The National Special Security Event plan takes a layered approach. Each NSSE plan is unique and includes multiple layers. One of these redundancies includes the outer perimeter fence erected to prevent intrusion into the inner area,” the statement said.

CPD remained on scene and cleared the area shortly after 6 p.m.

This is a developing story. Check back later for updates.