* Fans line up for hours to clear security checkpoints
* Queues also for water amid high temperatures
* Blast near cycling road race, suspected controlled explosion (Adds details, suspected controlled explosion)
By Karolos Grohmann
RIO DE JANEIRO, Aug 6 (Reuters) - The Rio Olympic Games got off to a shambolic start on Saturday as fans queued for hours at security checkpoints to enter venues, with some missing their events and many athletes competing in front of eerily empty stands.
Games organisers apologised for dropping the ball on the first day of full competition, the morning after a dazzling opening ceremony, as iconic venues such as beach volleyball on the famed Copacabana beach saw only a few hundred spectators.
Outside, lines stretched for several blocks as angry fans stood in full sun, waiting as security staff struggled.
"We apologise for everybody standing in line outside the venues," Games spokesman Mario Andrada told reporters.
"We need to upgrade that part of the Games. We moved people from Rio 2016 (organisation) to speed up the mag and bag (security checks) and within the next hours we will be in much better shape."
He said he hoped no fan had missed competitions they had bought tickets for, but many Brazilians were left fuming.
"I don't believe it. It's absurd, ridiculous," said Rio resident Natalia Carvalho, 28, who had wanted to see Brazilian gymnast and medal hopeful Arthur Zanetti compete.
Instead she entered the stadium only after he had finished his rings presentation with thousands more waiting to enter the Olympic park, site of several venues.
"I always said that what I most wanted to see was Zanetti in the rings. It's a lack of respect for the fan that bought tickets. It's a shame," she said.
Games organisers blamed the snafu on a lack of coordination between various groups of security personnel, including the police, Games staff and private security firms.
Organisers have applied tight security around the venues, amid concerns that the Games could be a target for militants. On Saturday, a bomb squad carried out a controlled explosion of an unattended bag near the finish line of the men's cycling road race. There have been several controlled blasts in recent days.
There were also some problems with transportation as buses spent hours trying to get people to the opening ceremony on Friday and some venues running out of water on Saturday.
At the tennis centre, where former world number one Ana Ivanovic played in front of virtually empty stands in the first round, fans had to wait more than 20 minutes to buy water.
Spectators encountered similar situations at the rowing and the rugby venues as temperatures hit 32 degrees Celcius (90 degrees Fahrenheit).
What should have been a celebration of the start of South America's first Olympics instead turned into a damage-control operation, with Andrada vowing an immediate improvement.
"It is the first morning of the Olympic crowd and some of the systems did not talk to each other. It was a problem of coordination. We need to explain to the public. We owe them an explanation and we owe them an excuse."
Only a few hundred spectators made it into the gymnastics arena, which can seat 13,500 people. The boxing venue also had many empty seats on Saturday morning as the first professionals in Olympics history entered the ring.
"We hope to clear the lines in the Olympic park," Andrada said, admitting some people had waited as long as 90 minutes.
"We feel sorry for the people who are there. Some time in line is acceptable but there needs to be a balance." (Additional reporting Pritha Sarkar, Liana Baker, Caroline Stauffer, Draz Jorgic, Pedro Fonseca and Alexandra Ulmer; Editing by Mark Bendeich)