The UFC champion Conor McGregor has promised to knock out boxing legend Floyd Mayweather inside four rounds when they faced off for the first time to promote their 26 August bout.

Fan favourite McGregor bullishly made the prediction during the pair's world promotional tour kicked off with fireworks in Los Angeles.

In front of 11,000 fans at LA's Staples Center, Mayweather and McGregor met face-to-face for the first time since their money-spinning megafight was confirmed.

Despite appearing in front of a home crowd and wearing a stars and stripes tracksuit, Mayweather was mainly booed by supporters, with the large Irish contingent cheering whenever McGregor appeared on the big screen.

McGregor said: 'On August 26, this man will be unconscious, he's too small, he's too frail.

'I will come out and paint many pictures. I can fight in my styles. I can dazzle him.

The 28-year-old - wearing a tailor-made expletive-laden suit - mocked Mayweather's tracksuit, 'little head' and 'little legs'.

But ever the showman, Mayweather pledged to 'kick ass' and told Mayweather he would be 'waving the white flag'.

'Popularity has nothing to do with what we've got in our bank accounts,' said Mayweather.

'He can be more famous.

'I always chose fortune over fame. You have a lot of people on social media with 20 million followers, but they may not have $20,000 in the bank.'

The boxing legend admitted, 'He's young, he's in his 20s, I'm in my 40s. He's active, I'm not.'

Mayweather revealed that the key to the fight taking place has been fan power.

At one stage, the chances of the duo locking horns appeared to be in the realms of fantasy, yet both Mayweather and UFC president Dana White praised the straightforward nature of the negotiations.

He said: 'I guess it was just something that the fans wanted to see.

The 40-year-old hasn't competed since beating fellow American Andre Berto on points in September 2015 and admits resuming his training schedule has been a shock to the system.