INDIANAPOLIS, AUG. 9 -- This first day of competition in the 10th Pan American Games found the United States baseball team battling for its Olympic life, a paddler from Rockville, Md., winning the Games' first medal, an anti-Castro group trying to help Cuban athletes defect and a Costa Rican swimmer giving her country unprecedented athletic joy and its first Pan American gold medal ever.

The U.S. squad avoided a possibly disastrous defeat in baseball by scoring three runs in the fifth inning and four in the seventh to erase a four-run deficit and defeat Canada, 10-6.

Norman Bellingham, 22, of Rockville, started the Games by winning the 500-meter kayak singles to begin the canoe/kayak events. He overtook Cuba's Luis Perez about halfway through.

Costa Rica, after going without a medal since 1951, won three today, including the nation's first gold medal ever. Leading the way for the tiny nation was swimmer Silvia Poll, who, with a time of 56.39, won the women's 100-meter freestyle. Later, Poll swam the anchor leg of the 800 freestyle relay, inheriting a 15-meter deficit before passing her Canadian opponent to secure second place and a silver medal. For her efforts, she was mobbed by her joyful teammates. Costa Rica's Ronald Lanzoni got things rolling this morning, winning a silver in the men's marathon.

Kelly McCormick, 27, the top women's diver in the 1983 Pan American Games, ran up a 10-dive score of 562.77 to better her old standard of 500.73, becoming the first woman to win successive three-meter springboard titles at the Games. But McCormick, of Columbus, Ohio, needed almost every point and great dives in the final two rounds to hold off teammate Megan Neyer, 25, of Boca Raton, Fla,. whose 544.32 earned her the silver medal.

The United States won as expected in men's basketball as forward Danny Manning scored 18 points to lead a 91-63 victory over Panama, which used five players who attend Briar Cliff College in Sioux City, Iowa.

Political controversy developed as the Cuban delegation sent a letter to organizers protesting what it said were provocations during opening ceremonies.

The Cubans complained of an airplane that flew over toting a banner with a telephone number for Cubans interested in defecting. Also, Spanish-language cards handed out said, "Cuban brothers, welcome to the land of freedom." The cards listed police and immigration telephone numbers.

The protest letter was sent to Mark Miles, president of PAX-Indianapolis, the local organizing committee, and Mario Vazquez Rana, president of the Pan American Sports Organization.

Theodore R. Boehm, chairman of PAX-I, said the letter was received this morning and "we're in the process of trying to determine exactly what happened, but I think we will conclude that it's simply the exercise of First Amendment rights of people with a particular view."

At Bush Stadium yesterday, a dozen Cuban-Americans, all of whom said they were veterans of the 1961 failed Bay of Pigs invasion, picketed while the Cuban baseball team played the Netherlands Antilles.

After the 12-1 win by Cuba, punches were traded between members of the Cuban delegation attending the game and some Cuban-Americans. There were no arrests.

The most significant victory of the day for the United States came at Bush Stadium. The U.S. team came into the opening baseball game with Canada needing a victory without which it would be extremely difficult to qualify for a spot in the 1988 Seoul Olympics.

The U.S. found itself trailing, 5-1, heading into the bottom of the fifth with only two hits off starter Rheal Cormier. Meanwhile, the Canadians had feasted on starter Greg Olson in three innings including the fifth, when right fielder Jim Kotkas hit a 420-foot home run over the 13-foot wall in left center.

The Americans had two careless base runners thrown out in the first inning, committed no fewer than three fielding mistakes in the outfield and easily could have been behind by more than four runs.

"It really was ugly," U.S. Coach Ron Fraser said. "But we showed a little something by putting it together when we had to."

The U.S. team is almost devoid of long-ball power, and it took a gust of wind and a shaky call by the second base umpire to get a rally going.

Scott Livingstone hit a pop fly that blew over Kotkas' head for a double. It appeared he overslid second base and was tagged out, but the umpire ruled that time had been called. Livingstone came around to score on two flyouts, cutting Canada's lead to 5-2.

The Canadians began to unravel. Slick-fielding first baseman Greg Duce committed a two-base error, allowing one run and setting up an RBI single by Mike Fiore.

The U.S. had struggled to get within 5-4 and nearly blew it when a blunder by left fielder Rick Johnston allowed a runner to score from third. But it didn't take long to wipe out the 6-4 deficit with a little help from a sudden burst of wildness from Canadian pitchers.

Tino Martinez, a left-handed first baseman who had four hits -- all off left-handed pitchers -- drove in two runs with a single up the middle. And two wild pitches mixed in with a run-scoring single by catcher Scott Servais put the U.S. ahead, 8-6.

Martinez supplied another two-run single in the eighth. But Fraser entrusted the rest of the game to short relief man Cris Carpenter, a man with a fastball so quick the St. Louis Cardinals are prepared to sign him as soon as the Games end.

Of the eight men he faced, two got hits but six struck out. "It's probably the best I've ever thrown in daylight," Carpenter said, alluding to the fact that his heat is almost unhittable at night.

Fraser, coach at the University of Miami, didn't low-key the importance of this game with his players. The U.S. rarely wins in international baseball and Fraser desperately wants this team to qualify for the 1988 Olympics.

To do that, the U.S. almost certainly must finish in the top three, perhaps in the top two, in these Games. And considering the dominant Cuban team is here, plus a Canadian team that recently beat the U.S. three out of five games, that is no sure thing.

Canada's Coach John Haar and pitching coach Ferguson Jenkins were upset with the way the game got away, but Canada is expected to be in the final four, and perhaps get another shot at the United States.

There was no such drama at most of the other venues in today's competition. Navy's David Robinson played sluggishly for the U.S. basketball team (11 points, 11 rebounds), but it hardly mattered against an overmatched team of Panamanians. In fact, the Americans had to be worried primarily about getting hurt by their teammates. Jeff Lebo of North Carolina and Fennis Dembo of Wyoming were knocked from the game after each took elbows to the face.

In track and field, Raymond Stewart of Jamaica ran a wind-aided, therefore nonrecord 9.89 in one semifinal of the men's 100-meter dash, .04 of a second faster than the world record.

One of the top U.S. sprinters, Chicago's Mark Witherspoon, finished second with a time of 9.92 but fell at the end and injured his right hamstring. U.S. Coach Tom Tellez said Witherspoon could run in the relays, but is "very doubtful for the rest of the competition."

Ivo Rodrigues of Brazil won the men's marathon with a time of 2.20:13, and Maricarmen Cardenas of Mexico won the women's marathon in 2.52:06. It wasn't an especially inspired marathon, by world-class standards; the times were slower because of the high winds that accompanied a morning storm.

Debra Warner, a 35-year-old graphic designer from Houston, was second in the women's division, even though she walked for awhile during midrace.

Cindy Greiner of the University of Oklahoma won the gold medal in the heptathlon, scoring comfortably ahead of Brazilian Orlane Santos.

The U.S. men's team handball squad won, 31-23, over Canada as Peter Lash of Charlottesville scored 10 U.S. goals.