Thirteen people were arrested during protests on the first day of the Democratic National Convention in Chicago, most them related to a “brief breach” of security fencing “within sight and sound of the United Center,” the city’s police superintendent said Tuesday.

More protests were planned throughout the week, including one Tuesday night outside the Israeli Consulate. However, attendance at the main rally on Monday was far below estimates of organizers who had predicted more than 20,000 would show up.

Chicago Police Superintendent Larry Snelling said Tuesday that the crowd was around 3,500 strong.