Protesters gathering in Chicago for the NATO summit are gearing up for their largest demonstration.

Thousands are expected to march later Sunday from a downtown park to the lakeside convention center where President Barack Obama and dozens of other world leaders will meet.

Several hundred demonstrators wound through the city's streets for hours Saturday. They increasingly tested police who used bicycles to barricade off streets and horseback officers to coax them in different directions.

Police said sporadic clashes led to 18 arrests.

Most of the demonstrations remained small. But a later march stretched for hours into Saturday evening as protesters zigzagged back and forth through the downtown and clashed with police lines.

Organizers pledged a larger crowd Sunday when Occupy protesters join forces with an anti-war coalition.