Katsuyama vs. O'Brien - who won the fight?

'Flash Boys' fight that stopped NYSE trading
'Flash Boys' fight that stopped NYSE trading

It was the fight that stopped trading on the floor of the New York Stock Exchange—IEX's Brad Katsuyama vs. BATS Global Markets president William O'Brien on high-speed trading and 'rigged' markets.

"You want to do this? Let's do this," Katsuyama said in response to O'Brien's prodding.

"I really do," O'Brien said.

(To watch the full 23 minutes of Katsuyama and Lewis vs. O'Brien, click here).

Read MoreMark Cuban rips high-frequency trading

And off they went, for more than 20 minutes. Traders stood transfixed as the two argued over the very nature of the stock market and whether the average investor could even compete with the big guys. (Twitter stopped too; see the tweets here).

(Watch: Michael Lewis on HFT)

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O'Brien pulled no punches in challenging Katsuyama and author Michael Lewis on their assertions that the market was unfair to ordinary investors.

"Shame on both of you for falsely accusing literally thousands of people and possibly scaring millions of investors in an effort to promote a business model," he said.

Read MoreMichael Lewis and why the market is rigged

O'Brien challenged Katsuyama to repeat his claims from Lewis's book "Flash Boys," and Katsuyama took the bait.

"I believe the markets are rigged and I also think you're part of the rigging," he said.

Read MoreLewis on the 'accidental conspiracy'

It wasn't just the NYSE floor, though, that was agape — "finance Twitter" also came to a standstill.

For many, it was the most intense thing they'd seen on the news in years.

Things also grew heated when Lewis was asked if he was secretly invested in Katsuyama's new exchange, IEX, as some have alleged.

Sue Herera: Are you an investor in Brad's exchange …
Lewis: No!
Herera: .. Michael?
Lewis: No!
Herera: I'm just asking for full disclosure. Call me kooky, I'm being a journalist.
Lewis: Absolutely not! Are you insane?
Herera: No, I'm not. I'm trying to do my job!
Lewis: I'm just trying to figure out what the hell is going on in the stock market. And big investor after big investor said there's one guy who's an honest broker in all of this. He's come into our office and explained how all of this works. Go talk to him. He's the only guy on Wall Street who can actually tell you the truth about the stock market.