President Joe Biden was seen doing a stage check Monday afternoon at the United Center in preparation for his opening night speech.

A march organized in support of Palestinians in Gaza during the DNC has started, hours before its first night of programming kicks off.

Organizers with the Coalition to March on the DNC, which represents more than 200 activist organizations, said about 15,000 people were present for a rally held in Union Park, near the convention site, ahead of the march.

Biden continues to "fine tune" his speech after working on it over the weekend and is in "great spirits" as he heads to the convention, White House press secretary Karine Jean-Pierre told reporters aboard Air Force One.

"Yeah, look, the president is certainly going to continue to fine tune the speech, if you will. He had an opportunity to work on it this past weekend with Mike Donilon and Vinay Reddy, and so he's very much looking forward to this moment. As I just said, it is a fulfilling moment for him, and he's excited. He's excited," Jean-Pierre said.

"It's a big night. He's looking forward to it. He's looking forward to addressing the Democratic Party and and obviously the nation. It's a big night," she added

Jean-Pierre said Biden was in a good mood on the plane ride over.

"He's in great spirits and I think what you'll see is a president who has put his kind of his personal agenda aside and putting the country first, and that's who he's always been throughout his career, more than 50 years," Jean-Pierre said.

Read more about what observers expect from Biden's opening night speech here.

-ABC News' Cheyenne Haslett

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