By Michael Bristow
BBC News, Beijing
Chinese officials have admitted that they are concerned about the lack of spectators at some Olympic events.
In a number of events, clumps of empty seats have been visible
Volunteers, dressed in yellow shirts, have been asked to fill up empty venues and improve the atmosphere inside.
But Wang Wei, a senior official with the Beijing organising committee (Bocog), said other Olympics had experienced similar problems.
The comments came after spectators and journalists noticed that certain venues were far from full, even though all events are sold out.
Speaking at a daily press briefing, Mr Wang said: "We are also concerned about this not full stadium [issue]."
He said a number of factors had contributed to this, including the hot and humid weather in Beijing, as well as the rain.
Mr Wang said some spectators were also only turning up for specific events, even though they had tickets for a whole session.
"For competitions like beach volleyball and basketball, [spectators] have one ticket for the whole afternoon, morning, evening," he explained.
"They may choose to go to one of them, but not all them."
Mr Wang, executive vice-president of Bocog, said local authorities were hiring volunteers to fill empty seats.
"If they find that there are not enough people, or if they find that there are too many empty seats, they organise some cheerleaders," he said.
These cheer for both sides to "create a good atmosphere", he added.
Although some events are full - such as Sunday's clash between the men's basketball teams from China and the United States - others have been less well attended.
These include sessions of judo, badminton and water polo.
"There were heaps of empties. It's sickening," said one spectator who went to the judo expecting to see a full house.
There were even a number of empty seats at the opening ceremony on Friday.
One reason for less-than-full venues could be that seats allocated to corporate sponsors are not being used.
Many of these tickets are handed out the night before events take place, sometimes too late for those who get them to attend, according to someone with access to these tickets.