TOKYO (AFP) - A chill wind is blowing through Japan's media as a resurgent right pressures the country's liberal voices, in what analysts say presents a threat to democracy as voters head to the polls for a general election.

Cheered on by fringe groups, the mainstream right-wing press and the government of nationalist Prime Minister Shinzo Abe are tightening the vice on left-of-centre voices.

A letter sent last month from Mr Abe's ruling Liberal Democratic Party to Tokyo-based broadcasters demanding "fair" coverage in the run-up to this weekend's vote sent a shiver through the industry. While the wording was tame, it awakened an institutional memory of 1930s Japan, when the press was strictly censored as military rulers marched the country towards war.

Coupled with the coming-into-force this week of a state secrets act that opponents say will stymie legitimate media criticism, campaign groups are warning of an increasingly febrile atmosphere.

The Japanese chapter of PEN International, an association of writers, has warned the law means the government "can now conceal military, intelligence and any inconvenient information in an arbitrary manner", in what it called the "return of the heavy-handed state".

Broadcasters and newspapers should fight back against attempts to muzzle them, said Mr Yasuhiko Tajima of Japan's Sophia University, one of several analysts to raise alarm over recent moves.

"Regardless of ideologies, right or left, the media's role is as a watchdog of power for citizens," he said. "Journalists shouldn't forget this principle - media is very important to democracy."

But it is in the realm of Japan's powerful multimillion selling newspapers where the right is making the most notable headway.

Their biggest scalp is the country's premier liberal newspaper, the Asahi Shimbun, which in August retracted decades-old articles on Japan's wartime system of sex slavery, after years of calls for it to do so.

The paper admitted its chief source for the stories was not credible - a fact long-since established - and apologised. The company's president also resigned.

Rivals leapt on the climbdown and Mr Abe, who wants a more sympathetic telling of Japan's history, took the move as proof of a smear.

"Japan's image was greatly damaged. Baseless libels are all across the world that 'Japan systematically forced sex slavery' because of the false (Asahi) reports," Mr Abe told a parliamentary committee in October.

Despite a dearth of official records, mainstream researchers estimate up to 200,000 women, many from Korea but also from China, Indonesia, the Philippines and Taiwan, served Japanese soldiers in "comfort stations".

Many historians say these women were not willing participants and that the Imperial Japanese Army and wartime government were involved in their enslavement, tacitly or explicitly.

Right-wingers, however, say the women were common prostitutes engaged in a commercial exchange.

While Mr Abe has shied away from withdrawing a 1993 Japanese apology for the system, he has ordered a review of it and makes plain his distaste.

But the Asahi's retraction has given a boost to the vociferous campaign by the right for that so-called Kono Statement to be annulled.

Sophia University's Mr Tajima said the government's attacks on the Asahi undermine its professed respect for human rights and for a free press.

"It's extremely rare for the leader of a country to name one media organisation and criticise it," he told AFP.

The Sankei Shimbun, a robustly nationalistic paper, and the right-wing Yomiuri Shimbun - the world's biggest newspaper with 10 million copies sold daily - devoted acres of coverage to the episode.

The Yomiuri published a book highlighting the error and accusing the Asahi of bearing "responsibility for harming Japan's national interest".

Last month, it issued its own pointed "apology" for what it said was the use of "misleading expressions" in its English language version, which had referred to "sex slaves". The paper believes it is not proven that the women were coerced.

An Asahi boycott campaign spearheaded by online groups has presaged a more than 500,000 drop in the paper's circulation, to around seven million currently, according to the Japan Audit Bureau of Circulations.

Professor Norikazu Kawagishi, a law professor at Waseda University, said the sight of Japan's right-of-centre media clubbing together with the government was a worrying one.

"Conservative media should be careful," he said, "because criticising the Asahi could deprive us of diversity in debates in our society".