Japan's government will Tuesday proclaim the right to send soldiers into battle even when the country is not under direct attack, the most significant change in military policy since the pacifist constitution was written.

Conservative Prime Minister Shinzo Abe is expected to crown months of political horse-trading when his cabinet formally endorses a reinterpretation of rules that have banned the use of armed force except in very narrowly-defined circumstances.

Despite widespread public opposition that climaxed at the weekend when a middle-aged man attempted suicide by setting himself on fire in Tokyo, Abe will invoke the right to exercise so-called "collective self-defence".

"The government has studied whether there is a defect in the current legal framework in protecting people's lives and property and Japan's safety... and we'll write the necessary legislation," top government spokesman Yoshihide Suga told a regular press conference.

"Naturally we'll have parliamentary debate in the legislative process, through which we will make detailed explanation to people," said Suga, the chief cabinet secretary.

While the move needs parliamentary approval to come into force, this appears to be largely a formality because Abe's Liberal Democratic Party controls both chambers.

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 supermajority 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 change what the clause means.

Under the new interpretation, 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.

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.

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

"I will continue efforts to explain (to the international community) that Japan's basic position to walk the path as a peaceful nation will not change at all," Foreign Minister Fumio Kishida told reporters.

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.

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.

While the anger has been palpable, Sunday's dramatic suicide bid, in which a protester doused himself in flammable liquid and set himself on fire near a busy Tokyo train station, was the most extreme example of the strength of feeling.

At a protest on Tuesday morning numbering in the hundreds, many voiced disquiet over both the change and the way it was being brought about.

"One cabinet can change the interpretation of the constitution? That's clearly illegal," said Yoshino Tani, the 38 year-old mother of twin five-year-old boys.

"I didn't give birth to children to send them to battlefields."

Japan's well-equipped Self Defence Forces, which were launched exactly 60 years ago Tuesday, have never fired a shot in a war, although they have conducted humanitarian missions.