The first mass rally during convention week in Chicago kicked off Monday afternoon in Union Park, located about four blocks east of the United Center, where the Democratic National Convention is being held.

Organizers with the Coalition to March on the DNC, which represents more than 200 activist organizations that gathered Monday in support of Palestinians in Gaza, said about 15,000 people were present.

ABC News spoke with several protesters ahead of the event. Many were native Chicagoans like Ken Justus, 68, who rode his bicycle, brandishing a "Swing Left" sign and sleigh bells. He said he still plans to vote for Vice President Kamala Harris.

"I want to make sure we still have a government," he said. His motto: "Vote blue no matter who."

Nearby, a coalition of nearly two dozen women from Indiana held banners to represent their organization, Jewish Voices for Peace. Their aim, said national board member Malkah Bird, 45, was to put pressure on the White House "no matter who is in office."

"We want our money reinvested in the U.S. to help our communities and not used to bomb Palestinians," Bird said.

One man, who asked ABC News not to use his name, said this was the first protest he had ever attended in his life. What motivated him, he said, was the "hypocrisy" of the administration's support of Ukraine while sending arms to Israel.

"If we want to be the leader of the free world, we need to exercise the responsibility of being the leader of the free world," he said.

Joshua Hansom, 52, traveled from Asheville, North Carolina, hoping to encourage people of all political stripes to come together through soccer. He passed cards to passersby encouraging them to join him to play soccer for two scheduled matches near Buckingham Fountain along the city's lakefront.

What will come out of it?

"Hopefully some solutions," he said. "At least some unity."

-ABC News' Mark Guarino

President Biden and first lady Jill Biden boarded Air Force One just after 2 p.m. ET to depart for Chicago.

They were followed by Peter Neal, the husband of Biden's granddaughter Naomi Biden.

Traveling with the president are key members of his staff, including chief of staff Jeff Zients and senior advisers Mike Donilon and Steve Ricchetti. White House press secretary Karine Jean-Pierre will gaggle with reporters during the flight.

Ashley Biden will introduce her father, President Joe Biden, tonight ahead of his remarks to the DNC, according to the schedule.

-ABC News' Isabella Murray

United Auto Workers President Shawn Fein, Rep. Jasmine Crockett of Texas and everyday Americans -- such as women who have been "subjected to cruel and dangerous abortion bans" -- will speak at the DNC Monday night, said Harris Deputy Campaign Manager Quentin Fulks at the committee's daily press briefing.
Rep. Alexandria Ocasio-Cortez of New York also announced in a post on X that she will be speaking at 9 p.m. ET.

A full rundown of the Monday's DNC convention speakers is still imminent, but several other names have been confirmed including President Joe Biden, first lady Jill Biden and former Secretary of State Hillary Clinton.

-ABC News' Isabella Murray