By Bradley Hope in New York, Chiara Albanese in London and Aruna Viswanatha in Washington 

In his first comments since being arrested for allegedly manipulating the U.S. futures markets and contributing to the 2010 "flash crash," the London day trader Navinder Singh Sarao said he had "not done anything wrong apart from being good at my job."

A judge ruled against granting Mr. Sarao, 36 years old, a lower bail. He was unable to post the GBP5 million ($7.6 million) bail because his assets in the U.S. and in offshore banking jurisdictions had been frozen, according to his lawyer, James Lewis. Describing Mr. Sarao as "a man of good character," he vowed to appeal to the High Court.

The U.S. Department of Justice has charged Mr. Sarao with criminally manipulating the U.S. futures market. The Commodity Futures Trading Commission has also brought civil charges against him for the same alleged acts. In court documents, the CFTC and DOJ said Mr. Sarao's alleged trading contributed to the flash crash.

Wednesday's hearing came as exchanges, regulators and law-enforcement officials in the U.S. face rising pressure to crack down on the market manipulation known as "spoofing."

Mr. Sarao's remarks in court Wednesday highlight a debate in the trading community about what constitutes spoofing and how to distinguish between illegal manipulation of markets and deft trading.

The CFTC is pushing exchanges to monitor futures markets for manipulative behavior and bring enforcement actions against rule breakers. Stock-market regulators also are ramping up efforts to identify and stop spoofers.

Meanwhile, a new civil case was brought against two traders, Heet Khara and Nasim Salim , who allegedly used spoofing tactics to manipulate the price of gold and silver on the CME Group Inc.-owned exchange from the United Arab Emirates.

"I'm in touch with CME officials and cooperating with them in the best of my capacity," said Mr. Salim. Mr. Khara couldn't be reached for comment.

The 2010 Dodd-Frank financial-overhaul law outlawed spoofing, but the practice has continued to plague the markets, traders said.

The tactic is essentially bluffing, with a trader putting large orders he or she doesn't intend to fulfill into the market to trick others into moving the price in a way that is advantageous for the spoofer. When successful, it allows the spoofer to continuously buy low and sell high by pushing the price up and down by relatively small amounts.

However, some traders have argued spoofing is ill-defined. They say there are legitimate reasons to place and cancel orders, such as market-moving news. Others have argued spoofing is an age-old practice that only hurts other traders, not genuine investors or companies.

In a letter to the U.K.'s Financial Conduct Authority, Mr. Sarao described himself as an "old-school" trader who used "intuition," quick reflexes and a computer mouse, according to court documents.

He said his trading was more legitimate than that of the high-frequency trading firms who use high-speed telecommunications lines, powerful computers and algorithms to buy and sell in the markets.

Critics say spoofing distorts prices. It is particularly bothersome to firms that make markets, a strategy where they simultaneously offer to buy and sell securities or futures contracts in the hope of capturing a tiny spread between those prices.

Hudson River Trading, a high-frequency trading firm based in New York that uses market-making strategies, developed a system to identify suspicious trading that could be spoofing and might cause the firm to lose money.

For instance, the system might send an alert when large orders are placed to buy a stock, then are suddenly canceled and apparently replaced by new orders to sell.

On an average day, the system finds 10 to 20 incidents that the firm considers highly suspicious and several hundred that are moderately suspicious, said Adam Nunes, Hudson River Trading's head of business development. The firm regularly flags suspicious activity to regulators and exchanges.

Hudson River's spoofing-detection system refers to potential manipulators as Cylons, a reference to a race of robots that rise up against humans in the TV show "Battlestar Galactica."

The Financial Industry Regulatory Authority, Wall Street's self-funded watchdog, has developed alert systems for suspicious trading in the stock markets, too.

Thomas Gira, Finra's head of market regulation, said many of the flashes of volatility in stocks--where prices rapidly swing in the blink of an eye--could be related to a predatory trader in the markets.

"What we're doing is catching those flash points during the day and running surveillance" on the apparent perpetrators, he said. "In many instances, it might be a spoofing attempt."

Investigations can often be challenging because of their global scope. Recent Finra cases have shown incidents in which groups from countries such as China and Russia attempt to manipulate stock prices. There have been cases where groups use multiple accounts to carry out their strategies, making it harder for regulators and exchanges to determine that one entity is behind the activity.

In the futures market, where Mr. Sarao was alleged to be manipulating a key futures contract tied to the S&P 500 stock-market index, surveillance for spoofing incidents falls to the exchanges and the CFTC.

The CFTC says it has been hamstrung by resource constraints, with a 150-member enforcement staff that is smaller than what the agency had before it obtained wide-ranging new authority under Dodd Frank. With the reduced head count, the agency has encouraged exchanges to pursue more suspicious activity and to refer more cases to regulators.

Since Mr. Sarao's case was filed, CME Group has come under fire for failing to bring his allegedly manipulative trading to a halt despite having contacted him about tactics used on its exchanges as early as 2009.

The CFTC has pushed the CME to do a better job in spotting spoofing and taking action more quickly, and it conveyed that message in a November review of the disciplinary programs of its exchanges.

"Spoofing has always been prohibited in our markets," a spokeswoman for the CME said in a statement last week, highlighting a "strong record" and "numerous cases and associated sanctions."

The CFTC also has pushed for the Justice Department to bring more cases alleging trading violations, according to people familiar with the matter, meeting with federal prosecutors in Manhattan, Brooklyn, New Jersey, Chicago and the Eastern District of Virginia.

CME has taken pains to show it is serious about spoofing. Last week, it denied Messrs. Khara and Salim access to trading on its platforms for 60 days for alleged spoofing. The CFTC followed on Tuesday with its own charges against the two men.

The CME identified the alleged conduct in February and soon after told Mr. Khara he was under investigation, according to the CFTC complaint. After Mr. Khara's broker suspended his access as a result of the suspicions, he opened another account with a different broker and coordinated similar trades with Mr. Salim, the complaint said.

A federal judge in Manhattan froze their assets Tuesday and scheduled a hearing for later this month.

Write to Bradley Hope at bradley.hope@wsj.com and Chiara Albanese at chiara.albanese@wsj.com

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