Japan on Tuesday loosened the bonds on its powerful military, proclaiming the right to go into battle in defence of allies, in a highly controversial shift in the nation's pacifist stance.

After months of political horsetrading and browbeating of opponents, conservative Prime Minister Shinzo Abe said his cabinet had formally endorsed a reinterpretation of rules that have banned the use of armed force except in very narrowly-defined circumstances.

"I will protect Japanese people's lives and peaceful existence. As the prime minister, I have this grave responsibility. With this determination, the cabinet approved the basic policy for national security," Abe told a press conference.

"There is a misunderstanding that Japan will be involved in war in an effort to defend a foreign country. But this is impossible. It will be strictly a defensive measure to defend our people."

Abe has faced down widespread public opposition to the move, which climaxed at the weekend when a middle-aged man attempted suicide by setting himself on fire.

Some 10,000 protesters gathered Tuesday near the prime minister's office to oppose the plan, Kyodo News reported.

While the move to allow so-called "collective self-defence" needs parliamentary approval, the control of both chambers that Abe's Liberal Democratic Party enjoys renders this a formality.

Abe had originally planned to change Article 9 of the US-imposed constitution, which was adopted after World War II and renounces "the threat or use of force as means of settling international disputes".

But unable to muster the two-thirds majority he needed in both houses and unlikely to get an endorsement from the public in the required referendum, he changed tack, using what opponents say is sleight of hand to alter what the clause means.

Supporters say the reinterpretation is necessary because of the worsening security situation in East Asia, where an ever more confident China is pushing its territorial claims and an erratic North Korea is threatening stability.

Under the new definition, Japanese troops will be able to come to the aid of allies -- primarily the US -- if they come under attack from a common enemy, even if Japan is not the object of the attack.

Examples pushed by the Abe camp have included a missile attack by North Korea on US forces in Guam; they say the shift would allow Japanese troops to shoot down the missile as it passes overhead, something presently not allowed.

- 'Horror of war' -

China has warned against the move, saying it opens the door to remilitarisation of a country that is not sufficiently penitent for its actions in World War II.

Just ahead of the announcement Beijing said Tokyo was guilty of "stirring up troubles on historical issues".

"It's only natural for us to wonder if Japan is going to change its path of peaceful development that it has long pursued after the Second World War," said a foreign ministry spokesman.

The Japanese administration rebuts this, saying the change will allow it to promote a notion which it has dubbed "proactive pacifism".

But that will not entail involvement in military adventures overseas, such as in Afghanistan.

"There will be no change at all in our principle not to allow the dispatch of forces abroad," Abe said.

"We shall never repeat the horror of war. With this reflection in mind, Japan has gone on for 70 years after the war. It will never happen that Japan again becomes a country which goes to war."

The move has received backing from Washington, which has long encouraged Japan to take on more of a role in a very lopsided defence treaty.

But it has caused anger at home, where the pacifism on which the constitution is built is an article of faith for many Japanese.

At least half the population opposes a more aggressive military stance, according to weekend newspaper polls.

Hundreds, and sometimes thousands, of people have turned out to protest against the change at various demonstrations over recent weeks.

Sunday's dramatic suicide bid near a busy Tokyo train station was the most extreme example of the strength of feeling.

"Listen to the voices of the people!" and "Do not destroy Article 9 of the constitution", protesters chanted Tuesday.

Housewife Chieko Hirano, aged 63, told AFP: "I have come here to stop Japan from becoming a country that can go to war.

"Article 9 is a jewel about which we can boast to the world and it should not be altered."