WASHINGTON -- President Obama warned overeager shoppers and greedy credit card companies alike on Friday to act responsibly as he signed into law a bill designed to protect debt-ridden consumers from surprise charges.

The White House staged a signing ceremony in the Rose Garden, an indication of the legislation's importance to Obama. Though opposed by many financial companies, the bill cleared Congress with broad support.

Obama made clear that he didn't champion the changes with the intention of helping those who buy more than they can afford through "reckless spending or wishful thinking."

"Some get in over their heads by not using their heads," the president said. "I want to be clear: We do not excuse or condone folks who've acted irresponsibly."

And yet, he said, for many of the millions of Americans, trying to get out of debt has been made difficult and bewildering by their credit card companies.

Nearly 80 percent of Americans have credit cards and half of those carry a balance, according to the White House. The Federal Reserve estimates the nation is some $2.5 trillion in debt, a figure that does not include home mortgages.

Obama said many people have gotten "trapped" because of the downturn in the economy that has turned family budgets on their heads. But, he said, "part of it is the practices of the credit card companies."

He criticized policies that allowed for confusing fine print; the sudden appearance of unexplained fees on bills; unannounced shifts in payment deadlines, interest charges or rate increases even when payments aren't late; and payments directed to balances with the lowest interest rates rather than the highest.

"We're here to put a change to all that," Obama said.

One part of the bill Obama did not publicly celebrate at the signing, a gun amendment. The measure by Sen. Tom Coburn, R-Okla., allows people to bring loaded guns into national parks and wildlife refuges.

The addition of the amendment to the bill -- and Obama's acceptance of it -- was viewed as a bitter disappointment for gun-control advocates.

They watched gun-rights supporters gain a victory from a Democratic-controlled Congress and a Democratic president that they couldn't achieve under a Republican Congress and president. Many blamed the National Rifle Association, which pushed hard for the gun law.

Democrats lawmakers and aides said they didn't have enough time to send the bill to the House-Senate conference committee -- where the gun provision could have been removed without a vote -- and still get the bill to Obama by the Memorial Day weekend as he requested.

The new credit card rules, which go into effect in nine months, prohibit companies from giving cards to people under 21 unless they can prove they have the means to pay the debt or a parent or guardian co-signs. A customer also will have to be more than 60 days behind on a payment before seeing a rate increase on an existing balance. Even then, the lender will be required to restore the previous, lower rate if the cardholder pays the minimum balance on time for six months.

And consumers also will have to receive 45 days' notice and an explanation before their interest rates increase.

Despite being touted as a victory for consumers, financial experts said the bill could have unintended consequences as credit card companies look for ways to make up for potential lost revenue. Those measures could include more cards with annual fees and the loss of a grace period before interest accrues, which would affect even those consumers who pay off their balance each month.

Last year, the Nilson Report estimated that more than 700 million credit cards were in circulation in the United States. That's more than two cards for every man, woman and child.

The president noted that nearly half of all Americans carry a balance on their credit cards, and that their average balance is more than $7,000.

Obama decried the "uneasy, unstable dependence" that a minority of card users have on credit.

"So we're not going to give people a free pass, and we expect consumers to live within their means and pay what they owe," Obama said. "But we also expect financial institutions to act with the same sense of responsibility that the American people aspire to in their own lives."