Protesters on both sides gathered outside the court from the early hours, and eager spectators formed long lines in hopes of a chance to watch the justices examine the opposing claims for religious freedom and gay rights.
The landmark case pits Dave Mullins and Charlie Craig against a Colorado bakery owner who refused in July 2012 to make a cake for their same-sex wedding reception.
Jack Phillips, owner of "Masterpiece Cakeshop" in Lakewood, Colorado, cited his devout Christian beliefs in turning down their request for a cake.
"What a cake celebrating this event would communicate was a message that contradicts my deepest religious convictions," Phillips wrote this week in USA Today.
"And as an artist, that's just not something I'm able to do... So I politely declined."
Mullins, a 33-year-old poet and musician, and Craig, a 37-year-old interior designer, said they were emotionally devastated by the rejection and filed suit for discrimination.
"This case is not about artistic freedom," Mullins told AFP. "We didn't ask for a piece of art.
"We were simply turned away because of who we were," he said. "We were publicly humiliated for who we were."
The Colorado Civil Rights Commission and the state appeals court backed up Mullins and Craig. And now it's in the hands of the nine-member Supreme Court, which has a conservative majority since President Donald Trump's appointee Justice Neil Gorsuch joined the bench.
Mullins, Craig and civil rights groups have warned that if Phillips is victorious, other businesses could cite religious beliefs to refuse service to gay customers.
"When you open a business to the public, you must serve the public equally," Mullins said.
"If a business owner is allowed to turn someone away just for their strongly held beliefs, could a hotel owner turn away an interracial couple because their faith believes that the races where not meant to mix?" he asked.
"A loss at the Supreme Court could open the door to many forms of discrimination that have long been considered wrong in our society."
'One man and one woman'
Phillips said he would happily sell Mullins and Craig a cake -- just not a wedding cake that would conflict with his conviction that "God designed marriage as the union of one man and one woman."
"Just as I shouldn't be able to use the law to force others to design something that promotes my beliefs, others shouldn't be able to force me to design a cake that celebrates theirs," he added in a USA Today interview.
Public interest in the case was evident in the long lines that formed for a chance at a seat in the hearing.
Hannah Boltz, an 18-year-old high school sudent from Maryland, spent the night outside with her classmates.
"We've been talking about this case in class," she said. "My moral side to it is that the gay couple should be able to get the cake. My constitutional side to it is that the baker has the right to refuse because it is his private business."
Jeremy Tedesco, an attorney for Alliance Defending Freedom, a Christian legal aid group representing Phillips, rejected the argument that the bakery owner had discriminated against the couple.
"Jack offered to sell the two gentlemen suing him basically anything in his shop," Tedesco told AFP. "Jack serves all who walk through his doors, no matter what their background or walk of life.
"But like other artists, he just can't create all messages for all events," he said. "Declining to express celebratory messages about same-sex marriages is simply not class-based discrimination.
"This case is about the freedom of artists and creative professionals to not be forced by the government to create custom expression or work for a message or event that violates who they are," Tedesco said.
"To say that a Muslim, Jewish or Christian artist can no longer adhere to the teachings of their faith and be forced by the government to create expression that violates their beliefs is scary," he said.
"Jack and his family have faced death threats and government punishment... In a truly free society, tolerance should be a two-way street."
Some 20 states, dozens of members of Congress and Christian lobbyist groups have thrown their weight behind the baker.
The Trump administration has also argued that the baker's cakes are a form of artistic expression and that he cannot be forced to use his talents against his own religious beliefs.
Among those representing Mullins and Craig is the American Civil Liberties Union.
"This isn't about a cake," said Louise Melling, the ACLU's deputy legal director.
"This is a question about whether the Constitution protects the right to discriminate," Melling said.
"It's about whether the Constitution protects the right of a bakery to put up in its store window a sign that says 'Wedding cakes for heterosexuals only.'"
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)