WEIGH-IN: Canelo Alvarez, left, and Gennady Golovkin pose during a weigh-in in Las Vegas. Golovkin's trainer says his fighter was insulted when Alvarez tested positive for a performance enhancing drug that caused their planned May middleweight title rematch to be postponed. Photo: John Locher/AP

Gennady Golovkin and Saul 'Canelo' Alvarez had to be separated at the weigh-in for Saturday's rematch in Las Vegas, with the world middleweight champion labelling his Mexican opponent a "clown" and promising "a real war".

Golovkin tipped the scales at 159.6 pounds and Alvarez came in at 159.4 at Friday's weigh-in, where the Mexican put his forehead against Golovkin's as they squared up to each other, prompting a brief shoving match on the T-Mobile Arena stage.

Their first fight last September ended in a controversial draw when each fighter got the nod from one judge, while the third declared it a tie, allowing Golovkin to retain his World Boxing Council, World Boxing Association and International Boxing Federation belts.

Golovkin is a slight favourite heading into Saturday's rematch.

"He is like a clown, he is a showman. He is not a true guy," said the 36-year-old Golovkin, who goes by the nickname "Triple G."

"Tomorrow is a real war. This is not a regular fight, this is a special war."

The bad blood between the pair has grown since Alvarez was temporarily suspended by the Nevada State Athletic Commission in April for testing positive for the banned substance clenbuterol, forcing a postponement of their planned May 5 rematch.

Alvarez blamed the positive test on contaminated meat but Golovkin has questioned the 28-year-old's character over it.

Alvarez said he got carried away by the atmosphere at the weigh-in.

"Look I got excited from seeing all the fans and it motivated me to do that right now," he said through a translator.

"I defeated him at the weigh-in, now it's time for me to defeat him on Saturday."

Golovkin is unbeaten with a 39-0-1 record, while Alvarez is 49-1-2, with his only loss coming against Floyd Mayweather in 2013.