Dressed in Wrangler jeans, a leather jacket, cowboy hat and boots, President Reagan strode into a dirt-filled corral at the Capital Centre yesterday--right behind two sequin-garbed cowgirls on horseback waving American flags--for a salute to the American rodeo and, as the announcer put it, "the men and women who have the moral fiber we need in this country."

By that he meant cowboys--and cowgirls--who were both in abundance at the Professional Rodeo Cowboys Association's "command performance" for the president. They rode bulls bareback, roped calves and wrestled with steers and got thrown up in the air like toy dolls--all to the cheers of the audience of 8,000 that also included members of Congress and the diplomatic corps, congressional staffers, White House aides and hundreds of employes of the federal government.

Last night, it was the president's turn to do the honors, as he hosted a barbecue dinner for 450 guests, mostly from the cowboy association, on the South Lawn of the White House. Red-and-white checkered tablecloths covered the picnic tables that held hand-tooled cowboy boots, substituting as vases for bouquets of fall flowers.

To the Western tunes of a group called Red Steagall and the Coleman County Cowboys from Fort Worth, the guests lined up for barbecued New York strip steak and chili beans, sourdough bread and cheesecake with strawberries.

Secretary of Agriculture John R. Block, wearing the cowboy garb that was the administration uniform of the evening, surprised the crowd by grabbing a guitar and proceeding to warble, almost on key, while his wife, Sue, held music for him.

"I didn't use to have the nerve to sing in public , till I came to Washington," Block said. "Now I'm not afraid of anything."

Reagan took the occasion to present a specially-struck Congressional Gold Medal to Louis L'Amour, author of 87 western novels, and hand out belt-buckle prizes to the winners of the rodeo events.

As he prepared to present the medal to L'Amour, the president was looking in one direction as the author came up behind him. "You sneaked up on me like Bowdrie," Reagan told L'Amour.

"I bet you were surprised I mentioned Bowdrie," Reagan told L'Amour later. Bowdrie is a Texas Ranger who is a character is several of L'Amour's books.

It was Americana all the way-just as it had been at the Capital Centre. The patriotic theme was apparent throughout the afternoon at the Landover arena--from the band that struck up "Deep in the Heart of Texas" as the president walked in, to the long poem in praise of the American flag ("The greatest flag in God's green country") read to the music of "America the Beautiful," to the tribute given Coors beer, Dodge Trucks, Wrangler, Coca-Cola and the other corporate sponsors for the event.

There was Vicki Tyler, who managed to get two horses to jump through a large horseshoe set on fire while she balanced herself on the horses' backs and kept the red tassles of her white cowgirl outfit blowing behind her.

There was Commerce Secretary Malcolm Baldrige, a longtime member of the rodeo association, who got into the act as part of a steer-roping team. But alas, the cowboy who headed Baldrige's team failed to lasso the steer, which is the object of it all, and the team scored no points.

Before Baldrige came out, the announcer quoted the secretary as saying he hoped he would do well at the rodeo because "all the time I've been in Washington, I've gotten one bum steer after another."

After it was over, Reagan said he had had "heartfelt thanks for these people who have entertained us so royally. There aren't too many activities in this land of ours that are as purely American as what we've seen here today." He then pronounced the rodeo "a national sport."

And the president was unable to resist his share of cowboy metaphors either. Speaking at the White House, he said: "Watching you out there today riding those bulls I knew I was among a group that understands what it's like to get a legislative program through," and added that "Secretary Baldrige has proved himself a multitalented cabinet member. Every time we come up to a real problem, he has the same solution: 'I'll rope 'em, you tie 'em.' "

Addressing the crowd at the White House, Baldrige said that after he came to Washington he was asked the difference between the rodeo circuit and the Washington circuit.

"That's simple," Baldrige said, "cowboys don't usually talk unless they've got something to say."

But members of various humane groups were not as effusive as Reagan about the rodeo's events. They greeted motorists turning into the Capital Centre yesterday with a picket line and large signs that read, "Real Men Don't Hurt Animals for $$$$" and passed out literature objecting to the rodeos, which they maintain abuses animals.

"The president is sanctioning cruelty to animals," said Karin Anderson, who passed out literature at one of the entrances dressed as a clown. She carried a sign that said, "Cruelty to animals is not funny."

The American Horse Protection Association, the Humane Society, the American Humane Association and the Fund for Animals chipped in and hired a small plane to fly over the Capital Centre streaming a banner that said, "Rodeo: Cruelty, Not Sport."

"I think they're probably right, but I like the rodeo. I'm from New Mexico and we have lots of it," said Mitchell Foushee, an aide to Sen. Jeff Bingaman (D-N.M.), who was in the audience.

The one break in the jovial atmosphere occurred when Charles Sampson, the 1982 world champion bull rider, was injured when he was thrown from a bull. Sampson was listed in good condition last night at Prince George's General Hospital after surgery for facial fractures.