“I have never seen it happen so quickly,” said Steve Walsh, a mortgage broker in Scottsdale, Ariz. “Banks always do these little cutbacks here and there. What they are doing now is a liquidity crunch. It’s a credit freeze.”

Richard F. Syron, chief executive of Freddie Mac, the large buyer of mortgages created by Congress in the 1970s, said yesterday that the speed and severity of the tighter credit terms are surprising, but perhaps necessary given the excesses in the market in recent years.

In a telephone interview from Washington, he was wary of the calls by some mortgage industry officials that Freddie Mac and its cousin, Fannie Mae, step in to buy loans and securities that private investors will no longer purchase. Mr. Syron noted that his company was operating under an agreement with its regulator that limited the size of its portfolio.

“There are some loans that are in difficulty” because credit pools are drying up, Mr. Syron said. “There are other loans that probably should never have been made and providing more liquidity will make that situation worse in the long term.”

The interest rates on many popular mortgages have risen by as much as a full percentage point, if they are available at all, said George J. Jenich, founder of FreeRateSearch.com, a consumer Web site. But rates on conventional fixed-rate 30-year mortgages have held steady.

Bear Stearns scheduled its conference call to reassure investors after Standard & Poor’s, one of the agencies that rates the creditworthiness of companies, said it was considering downgrading Bear’s credit rating because of the collapse of two hedge funds it recently put into bankruptcy after they made losing bets on mortgage securities.

Despite all the worries about credit markets, however, the economy continues to plow ahead and even yesterday’s jobs report was not weak enough to suggest that the Federal Reserve would cut its benchmark short-term interest rate when it meets next week.