Pop music singers Mariah Carey, Usher and Jennifer Hudson will mix with R&B veterans Smokey Robinson, Lionel Richie and Stevie Wonder while sports stars like Kobe Bryant and other celebrities such as Brooke Shields also are expected to turn out.

Some 18,000 fans and friends will crowd into the Staples Center sports arena and a nearby, overflow theater for the two-hour ceremony memorializing pop star Jackson, who died June 25 after suffering cardiac arrest in his Los Angeles mansion.

Police estimate more than 250,000 people will cram onto the sidewalks outside the arena to pay their final respect to the "Thriller" singer and one-time member of Motown legends the Jackson 5, who was 50 years-old when he died.

"This is certainly a momentous occasion that is probably as big, if not bigger than, when Elvis (Presley) passed away," said Steve Howard, a resident of Glendale, California, who won a ticket in an online lottery.

"The impact he had on American music and world music crossed all boundaries," said Howard, who expects the service to feature performances by Jackson's friends and fellow singers, along with eulogies for the fallen pop star.

Two people who will not be there are Jackson's ex-wife, Debbie Rowe, who said on Monday her attendance might be a distraction, and long-time Jackson friend actress Elizabeth Taylor, who said she was asked to speak but was too overcome by grief.

Media reports have said Jackson is expected to be buried in a private family service in Los Angeles ahead of the memorial, but a family spokesman declined to comment on that service.

Questions persist over who will pay for police security and other services such as sanitation required for such a massive gathering. Cost estimates were hard to come by, but Los Angeles City Councilman Dennis Zine estimated as much as $2.5 million.

Like other cities, Los Angeles is strapped for cash in the current recession, and people have complained that public money should not be used for what is, in some ways, a private event.

Still, acting Mayor Jan Perry has said police and other agencies have contingency budgets for events such as this.

About 1.6 million people registered to be among the 8,750 who won two free tickets to the event, and police expect many who did not win tickets to show up outside.

The memorial will be televised live on major U.S. networks, as well as streamed on the Internet.