Christian rock music blared through loudspeakers and pictures of Kirk were set on easels throughout the walkways of the State Farm Stadium in Glendale, which has a capacity of more than 63,000.
Crowds of people, many dressed in their Sunday best, had arrived before dawn to get inside to mourn Kirk, who was killed onstage in Utah on September 10 as he debated with college students.
Traffic jammed the surrounding roads and overflow space was set up at another arena nearby.
Trump, speaking to reporters before flying to Arizona, said he hoped the service would be "a time of healing" and spoke of his young political ally's influence, crediting him for building support for Republicans through his student activist group Turning Point USA.
"He did a tremendous job, and he had a hold on youth because they loved him," Trump said.
"If you go back 10 years, those colleges were dangerous places for conservatives and now they're hot. They're very hot, just like this country is hot."
Security was extremely tight, given attendance by Trump and several members of his cabinet as well as the ongoing political turmoil in the aftermath of Kirk's death.
People fanned each other in the heat as they waited to shuffle through metal detectors.
An older man dropped to the ground and was carried out of the line to get help.
Occasional arguments broke out over line-jumpers: someone yelled "liberals!" at a group that tried to move to the front.
Giant screens showed photographs of Kirk, including one in which he is leaning in to kiss his wife Erika who was elected as Turning Point's chief executive last week.
Kirk, 31, was killed with a single bullet as he answered an audience member's question at a campus event in Utah organised by Turning Point.
A 22-year-old technical college student has been charged with Kirk's murder, and investigators say he told his romantic partner in text messages that he had killed Kirk because he had "enough of his hate".
The killing has raised fears about the growing frequency of US political violence across the ideological spectrum while also deepening partisan divides.
The roster of speakers at Sunday's service, titled "Building a Legacy: Remembering Charlie Kirk," demonstrates Kirk's influence as the leader of the country's biggest conservative youth organisation.
In addition to Trump and Vance, who was friends with Kirk, senior administration officials including Secretary of State Marco Rubio, Secretary of Defense Pete Hegseth and Health Secretary Robert F Kennedy Jr will address the crowd.
In his remarks, Trump was expected to cast Kirk as a martyr for the conservative movement and highlight his legacy as a cultural and political force.
Kirk built a massive following through his savvy use of social media, radio shows and campus tours, where he often invited sceptical students to debate him, and was credited with mobilising young voters to Trump's cause in 2024.
Ken Skinner, 61, and his wife, with whom he runs a home daycare, drove nearly six hours from San Diego to attend the memorial; it took hours to get in.
He said he had tuned in to the livestream of Kirk speaking at the Utah college, and watched in horror as he was shot.
"I wish I didn't see it, but I did," he said.
"The devil came out in me. I was mad, just looking for someone to blame. But coming here, being with like-minded people, has helped."
Skinner said he hoped that Trump, whom he has voted for in three elections, "calms down the rhetoric and stops pointing fingers".
