More than a decade after an initial attempt failed, Japan wants to enact legislation to penalise conspiracies to commit crimes such as terrorism but critics say such changes would give police power to trample civil liberties.

Proponents say the steps are vital to fight terrorism in a climate where the risks have grown as Tokyo prepares to host the 2020 Olympics, and to ratify a U.N. Treaty aimed at battling international organised crime.

"It's very important ... to fight organised crime by cooperating with international society," Chief Cabinet Secretary Yoshihide Suga told reporters when asked about the proposals.

Opponents see the proposals as part of Prime Minister Shinzo Abe's broader agenda to tighten the government's grip at the expense of individual rights.

Critics including the Japan Federation of Bar Associations also warn the changes - when combined with a recent expansion of the scope of legal wiretapping and the reluctance of courts to rein in police surveillance powers - could have a chilling effect on grassroots opposition to government policies.

The federation also says Japanese law already prohibits preparing to commit certain serious crimes such as murder, arson and counterfeiting or plotting an insurgency or the use of explosives.

"The ruling coalition is emboldened and, led by the prime minister, wants to push these kinds of laws," said Meiji University law professor Lawrence Repeta. "He's pushing an agenda that grants the police and the government broader authority and limits the protection of individual rights."

Proposed changes floated in a document leaked to Japanese media and seen by Reuters would make it a crime for "organised crime groups" to conspire and prepare to commit serious crimes punishable by four or more years imprisonment.

Examples included gangsters readying a plot to shoot a rival gang leader and terrorists preparing a chemical attack. Workers chatting over drinks and saying they want to kill their boss wouldn't qualify, the document says.

"CHILLING EFFECT"

Legal experts, however, say the vague phrasing suggests that civic groups opposed to government policies could be considered "organised crime groups" if they are found to be preparing for illegal actions, such as plans by activists opposed to U.S. military bases blocking roads to construction sites.

"There is no clause limiting this to a group like the Mafia," said lawyer Yuichi Kaido. "It could target a group that is opposed to the government in some way."

If the revisions are enacted, "people will become afraid and it will have a chilling effect," he said.

Liberal Democratic Party-led governments have tried three times to enact such legislation since the United Nations adopted a Convention Against Transnational Organised Crime in 2000. But outcries from lawyers, activists and media scuppered the bills.

This time, though, Abe's ruling coalition has a two-thirds majority in both houses of parliament, and public worries about terrorism ahead of the Olympics have grown after deadly attacks overseas.

"We are facing the Tokyo Olympics and once something happens, it will already be too late," LDP Secretary General Toshihiro Nikai was quoted by media saying on a radio programme.

The justice ministry is taking pains to allay concerns ordinary people would be ensnared by the changes, stressing that's not true in a Q&A on its website.

Critics see the push for legal changes as integral to Abe's efforts, reflected in LDP proposals to revise Japan's post-war constitution, to strengthen the authority of the state.

Other moves include the 2013 enactment of a states secrets law toughening penalties on leaks, and changes this year that widened the scope for wiretapping to include fraud and theft.

Some legal experts find the trend especially troubling given the judiciary's reluctance to restrict police powers.

In a recent example, the Supreme Court this year declined to review a case concerning police surveillance of 17 Muslims who were monitored purely because of their religion, upholding a lower court ruling that the surveillance had been "necessary and unavoidable" to protect against terrorism.

"This would be added to an existing criminal justice system in which the courts do not serve as an effective check on police and prosecutors," Meiji University's Repeta said.