CONOR McGregor is making waves ahead of his UFC 246 return, with all eyes on the Irishman's stunning transformation.

The crowd erupted as McGregor emerged from behind the curtain and stepped up on to the scales to weigh in for the final time.

The ceremonial weigh-in took place at the Park Theater on the Las Vegas strip.

Self-help guru Tony Robbins and UFC heavyweight champion Stipe Miocic were among the faces in the front row while about 5000 fans packed in to the free event.

After igniting the crowd and officially weighing in, McGregor spoke to Joe Rogan and issued a blunt threat to the remainder of the division.

"I'm coming for all of them in this division. It starts with Donald!" he said.

The fight is set to officially go ahead on Sunday after both men weighed in at 170 pounds with McGregor and Donald Cerrone making the limit without fuss.

Both benefited from an agreement to not put their bodies through a major weight cut for this fight, but fans couldn't help but look on in shock at McGregor's stunning transformation.

The days of McGregor starving himself to make the 145-pound featherweight limit are long gone but it's still stunning to see how gaunt he looked during the early days of his career.

There's been a few flashback photos of McGregor this week, including this image from his coach John Kavanagh that shows them preparing for his first MMA fight.

Female fighter Alexa Grasso was the shock story of the weigh-in as she hit the scales at 121.5 pounds - well clear of the 115-pound limit for her fight against Claudia Gadelha.

READ: Ultimate guide to UFC 246

The bout was cancelled by the Nevada State Athletic Commission, which only permits a three-pound difference between combatants.

UFC 246 WEIGH-IN RESULTS - MAIN CARD

Welterweight - Conor McGregor (170) vs Donald Cerrone (170)

Bantamweight - Holly Holm (135.5) vs Raquel Pennington (136)

Heavyweight - Aleksei Oleinik (238) vs Maurice Greene (243)

Strawweight - Claudia Gadelha (115.5) vs Alexa Grasso (121.5)

Lightweight - Anthony Pettis (155.5) vs Diego Ferreira (155.5)

WHEN IS UFC 246 AND HOW CAN I WATCH?

UFC 246 takes place on Sunday, January 19 at T-Mobile Arena in Las Vegas. The early prelims start 10.30am AEST, with the main card commencing at approximately 2pm.

The early fights will be shown on ESPN from noon (AEDT) while the main card will be shown live and exclusively on Main Event.

Watch Conor McGregor vs Donald Cerrone on Sunday, January 19 (AEDT) only on Main Event. Order it now here