WASHINGTON (Reuters) - The United States was unprepared for the 2007-2008 financial crisis, underestimated its seriousness and lagged in coming to grips with the damage, past and current Treasury chiefs said yesterday.

Treasury Secretary Timothy Geithner told a commission investigating the causes of the crisis that it was vital now to tighten regulation of financial firms but not stop banks from taking prudent risks on behalf of customers.

In an apparent reference to a bid by some Senate Democrats to make banks spin off some derivatives trade, he said it doesn't make the economy safer to take risk-hedging activities outside of banks that offer the service to customers.

The commission is the latest hearing to rake over the aftermath of the crisis, as the battle over how to prevent a repeat heats up in the Senate ahead of November elections.

There was little of the political rancor though in more than four hours of polite questioning of Geithner and his predecessor, former Goldman Sachs chief executive Henry Paulson, and yesterday's session broke little new ground.

Both men conceded the state of readiness before the sub-prime mortgage-induced crisis hit was woeful.

Paulson said he was aware when he took office in mid-2006 that the potential for financial trouble was high but was taken by surprise by the severity of it.

"What wasn't clear to me..was the scale and degree of the problem," the gravel-voiced former Goldman Sachs head told the Financial Crisis Inquiry Commission.

"There wasn't a plan in place when when I arrived, I think we put a plan in place," he said. The Bush administration persuaded Congress to set up the $700 billion fund to bail out major banks, a controversial move that angered US taxpayers.

Geithner, who took over Treasury in early 2009 after serving in the powerful position of New York Federal Reserve Bank president, said there had been widespread complacency in the financial system that left participants and regulators thinking they could weather any storm.

"Financial crises are caused by the unwillingness of people to think the unthinkable...that mistake was pervasive across the system," Geithner said, adding it needs to be corrected and the mistakes of the past underline that necessity.

"If the government had moved more quickly to put in place better-designed constraints on risk-taking that captured where there was risk, the crisis could have been less severe and if the government had moved more quickly to deal with the damage this would have been less severe," he said.

Paulson said he thought most of his own mistakes were ones of communication, especially about the need for bank bailouts.

"I was never able to explain to the American people why these rescues were for them and not for Wall Street," he said. "I sure wish I'd communicated better a lot of the time."