Hundreds of left-wing protesters were preparing to march in Chicago on Monday as the Democratic National Convention got underway nearby, providing an early test of the city’s security preparations and of Vice President Kamala Harris’s attempts to project a sense of intraparty unity.

At least two protest marches were planned for the convention’s opening day, including by a coalition of more than 200 activist groups representing a range of causes that was expected to march within “sight and sound” of the United Center, the main convention hall, on Monday afternoon.

Activists carrying signs with messages like “Free Palestine!” began to gather for that event in Union Park on Monday morning, just west of downtown Chicago, and organizers were expecting thousands or tens of thousands of people to show up. Ellie Feyans-McCool, who traveled from Minnesota to attend the march, said she hoped protesters in Chicago this week would nudge the Democratic Party to withhold future military support for Israel.

“It is no longer good enough just to stand against Trump,” said Ms. Feyans-McCool, who added that she had not yet decided whether she would support Ms. Harris or a third candidate. “You have to do good.”

At one point, a group of about a dozen people with Israeli flags marched around Union Park, leading a small contingent of pro-Palestinian protesters to break off and march alongside them. The Chicago police, which had mostly kept their distance until that point, kept the two groups separated.