On the first day of the Olympic Games, Rodrigo Mascarenhas arrived at the Olympic Tennis Center ready for world-class competition and sold-out matches.

He found a crowd that would have seemed thin at a decent Little League T-ball game.

“I was surprised when I arrived and it was empty,” said the 37-year-old Brazilian, who works in public relations for Coca-Cola. “The tickets were sold out for tennis; you can't find it on the website. So I thought it would be crowded.”

Attendance, or the conspicuous lack of it, has been one of the trouble spots in the first days of Olympic competition in Rio. For lower-profile sports on Day 1, venues had swaths of empty seats, sections that stood nearly unpopulated, whole front rows free for the taking. Even for the prime-time events such as U.S. men’s basketball, Brazilian beach volleyball and the swimming races, fans were slow to their seats and the venues didn’t fill up. Organizers said they have not sold out a single event so far, apart from the Opening Ceremonies.

Even seemingly in-demand events had rows of empty seats when it came time for the action. Michael Brown, a booster of the U.S. women’s rowing team, had been frantically trying to buy tickets for Saturday’s races on a near-daily basis since they went on sale. After failing because they were supposedly sold out, he decided to stay home and not come to Brazil until later in the Games. Then he saw the venue on television.

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“They panned the stadium, and it was half-empty,” Brown, who is the president of a mining company and lives in Alexandria, said by telephone. “I would be there right now. I would have been there two days ago.”

The Olympics organizers blamed the shabby showing primarily on zealous security staff. Because of the intense fear of terrorism, entering any Olympic venue requires a procedure similar to boarding an airplane: bags and electronics into a scanner, people through a metal detector. This process caused long lines and huge delays.

“Some people, especially those with children, have decided to go home, which we regret,” Mario Andrada, a spokesman for Rio’s Olympic organizing committee, told reporters Sunday. When home team Brazil played beach volleyball on Copacabana beach, Andrada said that “we had an unacceptable line in the beginning” because of the security procedures, but that “this problem has been fixed and there’s very little waste of time to get into the venue.

“Even when we had some empty seats in some stadiums, we never had a low atmosphere,” he added.

According to organizers, the Olympics have sold 82 percent of their 6.1 million tickets and are running 5 percent above the target for expected income from ticket sales with $345 million in revenue. Three-quarters of Olympic fans so far have been Brazilians.

The logistics of getting to the stadiums was surely one hassle, but demand also seemed lackluster. Compared to the frenzied crowds during World Cup matches held in Brazil in 2014, the environment at the Olympic Park, and at outdoor viewing sessions elsewhere in the city, has felt tame.

Some Brazilians see the Olympics as a waste of money when their country is suffering through a severe recession, strikes by teachers and political upheaval that recently led to the president’s ouster. Protesters tried to block the arrival of the Olympic torch to Rio, and police fired tear gas and a concussion grenade at protesters on the day of the Opening Ceremonies.

“I have some friends that are very negative and think the Olympic Games are not important in this period,” said Mascarenhas, the fan at the tennis match. “Because we have a crisis, we have to spend money on other things like education and health. But I have a group of friends who are very excited, like me. It’s a mix.”

The threat of the Zika virus, which hit Brazil hard over the past year, may also have scared away some foreign fans. Several athletes pulled out of the Games citing concerns about Zika.

“I haven't even seen a mosquito. Knock on wood,” said Andrew Schwarz, 29, of Chicago, who made it to the first day of the Olympics despite a three-day delay in Miami. He was attending a women’s tennis match with his fiancee, Kristin Johnson, in a triple-decker stadium where 45 of the 48 front-row seats on one side of the court were unoccupied at one point.

“I commented today that I thought it looked pretty empty,” Johnson said. “I’ve never attended anything related, so I don't have much to compare it to.”

“But it seems crowded everywhere else we go,” Schwarz said.

When it came to lines, that was clearly the case. Beyond the security queues, lines with dozens and hundreds of people stretched back from different concession stands in the jumbo parking-lot-size courtyard of the Olympic Park. Organizers said that 150,000 people bought tickets for the Olympic Park events Saturday.

Another explanation was that Brazilians simply don’t have much experience watching people fence or shoot air pistols. They will live and die with soccer, and they love volleyball, with other sports lagging varying distances behind. Andrada, the organizing committee spokesman, said Brazilians were still “learning how to love and how to appreciate different sports.”

“Sometimes even we spot that they don’t know exactly how to behave in some sports because they’ve been fans of football and volleyball for many years,” he said.

On the second full day of sporting events, “we are now experiencing a daily acceleration in ticket sales as Brazilians are traditionally last-minute buyers,” a Rio organizing committee spokesperson wrote in an email. “We are continuing to promote ticket opportunities in less popular sports through a TV and digital media campaign under the slogan, ‘There is emotion in every place.’”

What Brazilians lacked in body count, they tried to make up for in enthusiasm. Andre Resende, attending the women’s beach volleyball match between the Brazilians and the Czechs, described the cheering atmosphere as “very good, very family, very excited, very safe.”

Asked about fans trickling off before their midnight beach volleyball match, three-time gold medalist Kerri Walsh Jennings said, “It felt like it was full. It doesn’t matter if it was half-empty. Those people cheered with all their hearts, and we appreciated it.”

Her playing partner, April Ross, added: “We’ll play for whoever wants to come.”

Jerry Brewer and Barry Svrluga in Rio de Janeiro contributed to this report.