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

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).

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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.

Read MoreMan vs Machine: Inside HFT

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