A surge in last-minute Christmas shopping helped lift retail sales out of the doldrums last month, but the nation's department stores still reported only modest sales increases for December.

Retailers noted that the gains over 1983 sales figures were less than they had anticipated, averaging about 7 to 8 percent instead of the double-digit percentage increases they had expected last spring when they made plans for Christmas, the biggest shopping season of the year.

What's more, retailers added, because of the large number of promotions launched near the end of the month in a last-minute attempt to lure shoppers, the increased sales will not translate into increased profits.

"As a result of the high level of promotional activity through the Christmas season, our fourth-quarter earnings are expected to be approximately equal to last year," predicted Kenneth A. Macke, chairman and chief executive officer of Dayton Hudson Corp. December sales for Dayton Hudson, the nation's fifth-largest retailer, were 15.8 percent above those rung up during December 1983, to $1.5 billion from $1.3 billion in 1983.

"The sales gains are satisfactory even though they are still below plan for most companies," said Monroe H. Greenstein, financial analyst with Bear Stearns & Co. "But because of the markdowns, earnings won't come close to plans," Greenstein added.

This will be particularly critical to retailers who expect to make half of their annual profits -- and a third of their annual sales -- during the Christmas season.

Among the biggest winners last month were K mart Corp., whose sales increased 17.1 percent to $3.6 billion from $3 billion a year earlier. K mart's chairman, Bernard M. Fauber, attributed part of the increase for the second-largest retail chain to its program to upgrade stores and merchandise.

Also doing well was Carter Hawley Hale Stores Inc., which saw its sales increase from $623.2 million in 1983 to $716.5 million last year -- a 15 percent rise.

"K mart and Carter Hawley Hale were extremely promotional, basically beating everybody's prices and taking greater markdowns than the rest of the industry," Greenstein said.

Other store chains did not fare as well. Sears, Roebuck & Co., the nation's largest retailer, saw its sales increase by only 4.7 percent for December, from $3.27 billion in 1983 to $3.43 billion in 1984. J. C. Penney Co., the third-largest retailer, posted a 6.8 percent increase, to $2.28 billion from $2.13 billion. Montgomery Ward, ranked sixth, saw sales increase by only a tenth of a percent for the month, to $1.03 billion from $1.025 billion.

The only retailer to report a decrease in sales was G. C. Murphy Co., which said its sales dropped 0.7 percent from $152.3 million to $151.3 million for the month. A company official said Murphy enjoyed an exceptional Christmas season in 1983, which it didn't expect to match.

In the case of Sears and Ward, some of the slow growth may be because of the reduced demand for large appliances, noted Stacy Ruchlamer, an analyst with Shearson, Lehman/American Express Co.

Warm weather also hurt these two retailers, whose automotive supply divisions account for a large part of their bottom line. Without cold weather, few consumers were buying batteries and other automotive supplies typical of the winter season, analysts said.

For all retailers, last-minute shopping led to record sales just before Christmas, Ruchlamer said. Until the last week, "sales were very, very depressed -- the consumer was still on his summer vacation," she said. But then sales picked up dramatically, "and carried through the last week. The big question is what will happen in January. Will the surge continue or will it turn south again?"

Results for other retailers include:

* May Department Stores Co. -- parent of The Hecht Co. -- reported a 10.8 percent increase in sales from $778 million to $862 million.

* Federated Department Stores, the nation's fourth-largest chain, said December sales grew 10.3 percent over 1983, from $1.6 billion to $1.7 billion.

* F. W. Woolworth Co. posted a 3.4 percent increase in sales, to $971 million from $939 million. Although the company's domestic sales were strong, it was hurt by its foreign operations, where the strengthening U.S. dollar reduced the amount of business Woolworth's did.

* Bradlees Department Stores, a division of The Stop & Shop Cos., had an 11 percent sales increase, from $250 million to $278 million.