Floyd Mayweather won a 12-round majority decision over Marcos Maidana to run his perfect record to 46-0 in a welterweight world title unification bout.

The 27-year-old American Mayweather retained his World Boxing Council world title and took Maidana's World Boxing Association belt, winning the fight on two of the three judges' scorecards with the third scoring it a draw.

"It was a tough competitive fight tonight," Mayweather said. "I stayed in there. I wanted to stand and fight."

Mayweather, who was an 11-1 favourite heading into the fight, received scores of 117-111 and 116-112. The third judge scored it 114-114.

Mayweather did not get the decisive victory that so many had predicted as the more aggressive Maidana gave him all he could handle.

Argentina's hard-punching Maidana, who was making the first defence of his title, suffered the fourth loss of his career to go with 35 victories.

Maidana was upset with the way the fight was scored, saying he was "robbed" of the decision.

"I definitely think I won," Maidana said. "He didn't fight like a man.

"He did dominate some rounds, but I dominated more of them."

Most of Mayweather's recent fights have been boring tactical affairs, but Maidana's relentless pursuit of the unbeaten champion and willingness to go toe-to-toe made for an exciting bout.

Many in the crowd also thought Maidana had done enough to win the fight and his supporters in the arena booed the decision then tried to drown out Mayweather's post-fight interview.

Mayweather made a grand entrance befitting his superstar status. He was accompanied into the ring by pop star Justin Bieber and rapper Lil Wayne.

Some of the other celebrities in attendance at Saturday's fight at the Grand Garden arena inside the massive MGM Grand hotel-casino complex included rapper Drake, former heavyweight world champion Mike Tyson, entertainer Snoop Dog, comedian George Lopez and basketball legend Magic Johnson.

The first few rounds were electric as Maidana stuck to his pre-fight promise to come out swinging. Mayweather seemed to be content to lean on the ropes and absorb the overhand rights that got the crowd excited but appeared to do little damage early on.

Mayweather suffered a cut over his right eye in the fourth round from an accidental head butt.

"After the head butt I couldn't see for two rounds. But that is what champions do, they adjust," Mayweather said.

Both fighters resorted to some questionable moves later in the fight.

Maidana hit Mayweather with a low blow in the eighth and later in the round Mayweather tried to surprise Maidana with a ghost punch while the Argentinian was trying to communicate with the referee about something Mayweather was doing that he didn't like.

In the 11th, Maidana pushed Mayweather partly through the ropes then threw more punches while Mayweather was leaning over backwards almost falling out of the ring.

Maidana landed a total of 221 punches -- more than any fighter against Mayweather in his last 38 fights.

Asked if he would give Maidana the rematch that the Argentinian says he wants Mayweather said, "Let's do it again."

Some see Mayweather's inability to dominate like he has done in the past as a sign that father time might be catching up to him.

"He is getting older and showing a lot of mistakes that he didn't used to do. It is a great time to catch him," said British boxer Amir Khan who beat Luis Collazo on Saturday's undercard.

Khan won a unanimous decision over former champion Collazo in his first fight after moving up to the welterweight division.

Khan, who was fighting for the first time after a 12-month layoff, knocked Collazo down three times en route to winning the non-title fight.

Khan's impressive showing may put him in line to fight Mayweather next.