DETROIT (AP) _ Sales of domestically-built cars and trucks rose in mid-August despite reports of declining consumer confidence, according to figures released Tuesday.

Car and light-truck sales rose 11.8 percent for the 10 major domestic automakers in the Aug. 11-20 period. Truck sales, which have been strong for months, showed a 31.2 percent increase. Car sales remained flat with a 0.2 percent decrease.

Leading the pack was Ford Motor Co., which reported a 23.6 percent overall increase. Ford truck sales increased 36.7 percent and car sales rose 13.7 percent from the same period a year ago. The automaker was the only one of the Big Three to report an increase in car sales.

''There's just more people coming through the door right now,'' said Bill Simmons, general sales manager at Dick Strauss Ford dealer in Richmond, Va.

''Our main problem with sales in August is that our inventories are starting to get slim,'' he said. ''We had assumed that this year things would be the same as last year, and now all of a sudden everything's getting better.''

The projected annual rate for new car sales rose to 6.2 million for the period, up from a three-month low of 5.8 million in early August. However, the Conference Board reported Tuesday that consumer confidence was down in August to its lowest level since March.

The widely watched economic survey said consumers were more negative than in July, both in terms of their view of the current situation and expectations for the immediate future. Since June, the index has fallen about 20 percent.

Analysts said that can only be bad news for automakers.

''There's been a recovery in the auto market, but it's a slow recovery - much slower than we and a lot of others expected,'' said Steve Girsky, an analyst with PaineWebber Inc. in New York.

''You have economic uncertainty, you have political uncertainty. All that doesn't make consumers want to go out and buy a new car ... especially after just paying off the last five-year car loan.''

There were nine selling days in this year's Aug. 11-20 period compared with eight last year.

General Motors Corp. reported a 2.9 percent overall increase in sales. But that was buoyed largely by a 19.3 rise in truck sales. Car sales fell 6.9 percent from this period last year.

Chrysler sales, estimated by Ward's Automotive Reports, rose 18.8 percent. Again, truck sales accounted for the bulk of the growth, with a 36.7 percent jump. Car sales fell 9.3 percent.

''We're starting to see more traffic and people are more willing to buy,'' said Chuck Cesario, sales manager at Herb Adcox Chevrolet-Geo in Chattanooga, Tenn.

''I think as soon as we get a president and get this election over with things will be even better. People won't be as worried,'' he said.

Mazda and Nissan both departed from the overall trend, reporting higher car sales and lower truck sales.

Nissan car sales were up 49.2 percent, and Mazda car sales rose 40.6 percent. Truck sales for the companies were down 12.8 percent and 8.7 percent respectively.

Such wide swings in sales percentages are possible among Japanese companies that build vehicles in the United States because this share of the overall market is so small compared with the U.S. Big Three.

Mazda's overall sales jumped 31.9 percent, and Nissan's showed a 13.2 percent increase.

Isuzu's truck sales were up 82.8 percent compared with this period last year. Toyota reported a 9.8 percent rise in car sales and 35.4 percent overall increase. American Honda's car sales dropped 16.5 percent for the period.