(Adds details of reporter detentions and Rogge visit in paragraphs 2 and 12-14)

By Nick Mulvenney

BEIJING, Aug 6 (Reuters) - Preparations for next year's Beijing Olympics are on track, organisers said on Monday, shrugging off concerns about food safety, pollution and accusations it has not lived up to its media freedom pledge.

Olympic chief Jacques Rogge said he believed the Games would be a significant force for good in China, but at a protest in Beijing calling for greater media freedom several journalists were briefly detained by police.

All Olympic venues -- with the exception of the showpiece "Bird's Nest" stadium -- would be finished by the end of the year and ticket sales were going well, said Wang Wei, executive vice president of the Beijing Organising Committee for the Olympic Games (BOCOG).

"Generally speaking, preparations for the Beijing Olympics are smoothly marching forward in accordance with plans," he told a news conference ahead of Wednesday's start to the one-year countdown to the Games. "City traffic conditions and the environment have got better."

Jiang Xiaoyu, also a BOCOG vice president, said regulations introduced at the start of the year to ease reporting restrictions for foreign journalists had been well received.

"We have already heard many different voices from many different sides, and we are mentally prepared for these voices to get louder and even reach a crescendo," he said.

"The Beijing Organising committee welcomes reporters from around the world to objectively, fairly and comprehensively report on the Olympic preparation work," Jiang added. "We welcome even more constructive criticism on faults and problems.

"But we absolutely oppose the politicisation of the Olympics, as this does not accord with the Olympic spirit."

Reporters Without Borders said on Monday China had made specific promises when it was awarded the Games that it would improve press freedom and human rights.

"They made very concrete promises, but unfortunately, six years after, we have the impression that there are no real changes," group secretary-general Robert Menard told reporters from a building opposite BOCOG headquarters, urging China to release all political prisoners.

"It is the Chinese government that has taken hostage the Olympic games because it does not respect its own commitment."

Menard urged Rogge, the President of the International Olympic Committee who was also visiting Beijing, to press China to release political prisoners and lift censorship.

"We believe the Games are going to move ahead the agenda of the social and human rights as far as possible, the Games are going to be a force for good. But the Games are not a panacea," Rogge told Reuters in an interview on Monday.

Several reporters present when four Reporters Without Borders representatives issued their call were kept from leaving by police for up to two hours with no explanation.

China has said that foreign reporters are free to report without restriction throughout most of the country in the lead-up to and during the 2008 Games.

China's censors cut off the signal on Monday morning for a live CNN broadcast from Beijing when the issue of press freedom came up.

FOOD AND AIR WORRIES

With the city once again shrouded in smog on Monday, Wang reiterated Beijing's work on improving air quality and, without going into particulars, said organisers would put special measures in place next August.

"Good air and blue skies are important for the Games," he said. "What matters more to us is not the image, but the health of the athletes and visitors during Games time."

Wang also insisted food safety would not be a problem.

"We have especially established a committee on food safety," he said. "... We have a lot of confidence we can ensure food safety in Beijing."

A range of Chinese exports, from fish and toys to pet food and toothpaste, has been found to be mislabelled, unsafe or dangerously contaminated, creating an international backlash.

Apart from pollution, food safety and media freedom, Jiang was clear what he thought was the main obstacle to hosting a successful Games Aug. 8-24 next year.

"The biggest challenge, I think, is how to make sure service levels and the cultural level and quality of our city and citizens matches the holding of a good Olympics," he said.

"We're very confident coping with these challenges and solving problems." (Additional reporting by Ben Blanchard and Chris Buckley)

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