(MSNBC) — Protesters gathering in Chicago for the NATO summit were gearing up for their largest demonstration Sunday, when thousands are expected to march from a downtown park to the lakeside convention center where President Barack Obama and dozens of other world leaders will meet.

Hours before the main demonstration was set to start, protesters were already gathering at Grant Park holding signs including one that read: “NATO, Go Home.” One protester walked with an American flag turned upside down.

A day earlier, several hundred demonstrators wound through the city’s streets for hours, testing police who used bicycles to barricade off streets and horseback officers to coax them in different directions. Increasingly tense clashes Saturday night between protesters and police resulted in 18 arrests, Police Supt. Garry McCarthy said.

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