American Motors Corp., its estimated early January sales down more than 28 percent from last year, today announced it was cutting 10 percent off suggested retail prices of all car models.

The reductions apply to all domestically built cars ordered or delivered to retail customers from Jan. 15 to Feb. 20, AMC said.

Two-thirds of the cut will come from a reduction in the prices AMC charges its dealers, and one-third will come from a reduction in dealer markups, AMC said.

The company said retail price cuts range from $497 to $559 on Spirits, $582 to $699 on Concords and $599 to $879 on four-wheel-drive Eagles.

AMC's sales for the first 10-day selling period in January were estimated at 2,600, representing a decline of 28.3 percent. AMC reports sales only at the end of each month.

Last week, General Motors Corp. raised prices an average of 1.5 percent, or $149, on its larger cars, cut $100 off the suggested retail price of its subcompact Chevette and froze prices on its compact X cars.

At the same time Ford Motor Co. raised prices an average of $85 on most models, froze prices on the Granada, Cougar and one Escort model and cut prices on one model each of the subcompact Escort and Lynx cars.

Chrysler Corporation has frozen all 1981 car prices.

Meanwhile, U.S. auto sales continue their downward trend. In Detroit today, automakers reported that domestic car sales in early January declined 19.8 percent from an unusually strong period last year.

Automakers said dealers sold 128,041 U.S.-built cars in the first 10-day selling period of 1981, compared with 159,628 in the same period last year.

That amounted to a seasonally adjusted annual selling rate of 7.2 million, well above December's poor rate of 6.1 million.

The improvement had been anticipated. One Big Three sales analyst said December is being viewed as the bottom of the sharp downturn that began in November with rising interest rates.

Chrysler Corp., its sales propped by rebates of 7 percent off the sticker price, continued to outperform the rest of the industry on a percentage basis and was the only domestic automaker to outsell last year's Jan. 1-10 period.

Chrysler reported sales of 16,179 U.S.-built cars, up 4.7 percent from 15,458 in the same period last year.

General Motors Corp. sales of 81,845 cars were down 17.2 percent from 98,883, and Ford Motor Co. declined 33.1 percent to 24,930 from 37,271.

Volkswagen of America recorded the industry's sharpest decline, falling 43.3 percent on sales of U.S.-built Rabbit subcompacts to 2,487 from 4,388 last year.

Chrysler said sales of its K car compacts improved to a daily rate of 1,053 units, up from 932 for December, and are now close to expectations.

GM's Chevrolet Chevette also saw some improvement over rates that forced the company to trim production of subcompacts. Chevette sales in early January were off 8.9 percent compared with a dropoff of 26 percent last month.

Ford said its new Escort subcompact was the nation's No. 2 selling car in December.

"That is an unprecedented performance by a newly introduced product," said Bennett E. Bidwell, vice president of Ford's car and truck group.