Olympic athletes have been competing in near empty venues and before thin crowds in other locations during the opening days of the Rio Games — a problem that can be traced to long security lines, traffic and a lack of familiarity in some sports.

Rugby and equestrian venues have been devoid of fans, and a Brazil volleyball match on famed Copacabana Beach had lots of empty seats Sunday. Scattered seats were open early during the basketball game Saturday night between China and star-studded Team USA, usually one of the biggest draws of the Olympics.

Rio organizers said long lines at airport-style security checkpoints prevented fans from getting into events. The line for the beach volleyball venue stretched down the Copacabana promenade to the Olympic rings display several blocks away, though the venue has not been full for any of the matches.

Organizing committee spokesman Mario Andrada the problem was being addressed, and he's hopeful that lines would move quicker as the Summer Games entered its first full week.

"We believe that most of the issues ... have been solved," Andrada said Sunday. "That doesn't mean we rest ... Security overall remains our main priority, and we're going to keep an eye on it."

There were long lines at bus stops on Saturday, the first full day of the Games. Lines at concessions stands were slow at the Deodoro venue, where events like rugby and shooting are being held, in large part because workers were stuck on security lines to get in the grounds.

"But I must emphasize, even when we have some empty seats in the some stadiums, we never had a low atmosphere," Andrada said.

Crowds were loud and enthusiastic for fencing. Swimming made a splash, as usual. The stands were nearly full with cheering fans when the U.S. women's gymnastics team took the floor for preliminaries on Sunday.

Boxing and rugby didn't fill the stands, though those who were there were vocal. Brazilian fans led soccer-style chants at the U.S.-China basketball that brought a lively atmosphere at times to the blowout.

"I'm a bit bummed to be honest, because especially these women have worked really hard to get here," said Nikki Diggs, a fan from California who watched Sunday's rugby match between the United States and Australia. The Olympics mark the first time women's rugby has been in the games.

"It's their first Olympics so it's a bit of a bummer but I think a lot of people were scared off and they shouldn't have been because everything has been really great," Diggs said.

Any problems with security lines seemed to have dissipated at the main Olympic Park venue by Sunday afternoon.

"We literally got out of a cab, walked in. Breezed right in," said fan Rochelle Taylor of Detroit. She and her friend waited in a security line for an hour on Saturday.

Security isn't necessarily the only reason that could be keeping people from their seats. Logistical issues can often slow things down early in an Olympics.

Traffic has been a problem, so much so that Mayor Eduardo Paes has pleaded with locals to share car rides until Aug. 21.

Andrada said about 82 percent of the 5 million tickets for the Olympics were sold as of Sunday. About 75 percent attendees are Brazilian.

Hoping to boost slow sales, organizers in mid-July opened up their local ticket website to the rest of the world, meaning that fans outside Brazil could buy tickets at local rates.

As for Brazilian fans, they're getting exposed to sports other than their beloved soccer and volleyball.

"Sometimes we spot that they don't know exactly how to behave in some sports because they've become fans of (soccer) for many years," Andrada said. "And one of the legacies of these games is to break the mold of either volleyball or football, for people to assess different sports."