Friday's service will be attended by Hawking's family, friends and colleagues and about 1,000 members of the public who were selected by a ballot.
Hawking's first wife Jane and daughter Lucy were among an eclectic crowd that included scientists and schoolchildren; politicians including U.K. Culture Secretary Matt Hancock and Labour Party leader Jeremy Corbyn; Chic guitarist Nile Rogers; actress Lily Cole; comedian David Walliams; and talk-show host Piers Morgan. There is no exact indication when the recording with Hawking's voice will be beamed towards the black hole.
Benedict Cumberbatch, actor, read Wisdom 7: 15-24, and Lucy Hawking, daughter, read Romans 12: 9-21.
His ashes were buried between Charles Darwin and Isaac Newton and later covered with a gravestone - etched with an equation he used it to teach the world about black holes. "Stephen Hawking proved that more than anyone".
Hawking lived with motor neurone disease, a debilitating neurological illness, and lost the use of his vocal chords in the 1980s.
Astronaut Tim Peake and two of Hawking's oldest professional friends, cosmologist Martin Rees and Kip Thorne, an American physicist and Nobel Prize victor, will also be in attendance.
A black slate memorial stone lies above, bearing the inscription: "Here lies what was mortal of Stephen Hawking 1942 - 2018".
"We are so grateful to Westminster Abbey for offering us the privilege of a Service of Thanksgiving for the extraordinary life of our father and for giving him such a distinguished final resting place", Hawking's children said in a statement.
Hawking died in March aged 76, with his funeral held in Cambridge, the city where he studied and worked.
He also hypothesized that miniature black holes, remnants of the big bang, may be strewn through space, though he noted that they haven't been discovered. I thought no one would take me but Richard Branson has offered me a seat on Virgin Galactic, and I said yes immediately.