Nationalist prime minister Shinzo Abe set out his case Thursday for beefing up pacifist Japan's rules of engagement, saying he wants the armed forces to be able to enter battle in defence of allies.

Citing a rising climate of disquiet in north and southeast Asia, Abe said Japan needs to cast off constitutional strictures that have prevented its so-called Self Defence Forces from firing a shot in combat since 1945.

"As prime minister, I have the responsibility to protect the lives of people under any circumstances," he told reporters in Tokyo.

"I don't think the constitution says we have to abandon the responsibility to protect the lives of people.

"If we can enhance our deterrence, it will prevent our country from being involved in war."

Around 500 people demonstrated against the prime minister's plans near his official residence, with some carrying banners that read "Exercising collective defence is equal to waging war".

The prime minister has long nurtured a desire to see more flexibility in Japan's pacifist constitution, which was imposed by the occupying United States in the aftermath of Tokyo's World War II defeat.

Article 9 of the document -- which has reportedly been nominated for the Nobel Peace Prize -- says Japan forever renounces the use of force as a means of settling international disputes.

For decades, governments have held that this means Japan's military may only open fire if fired upon, even if that entails leaving US counterparts in danger on the same battlefield.

Unable to change the constitution because of deep domestic resistance, Abe has argued for the next-best thing: a reinterpretation of the laws to permit "collective defence".

A panel of academics, diplomats and military advisers convened by the prime minister has come up with a series of proposals on possible legal frameworks for military action.

Over the coming months, Abe will use this document to persuade a sometimes-sceptical public of his case as he looks to shepherd his plans through the labyrinth of Japan's political system.

The move is controversial and risks forcing a split with his ruling party's coalition partner, New Komeito, secular Buddhists without whom Abe does not have an outright majority in the upper house of parliament.

"It will be the first reinterpretation of the constitution by a politician in Japan," said Tomoaki Iwai, professor of Japanese politics at Nippon University in Tokyo.

"It's going to be a turning point in the country's politics," Iwai said, adding he expected the formal reinterpretation that Abe wants by the end of the year.

- Memories of an imperialist past -

Voters are lukewarm on the idea; a poll of more than 2,000 adults nationwide showed 63 percent oppose the concept of collective defence, the Asahi Shimbun reported last month.

That was up from 56 percent last year and more than double the 29 percent who support the idea, the poll showed.

Abe wastes no opportunity to remind his audience, both at home and abroad, of Japan's track record since 1945.

"We have consistently walked on the path of pacifism for 70 years after the war and there will be no change to this," he said Thursday.

Despite this repeated reassurance, his drive to strengthen the military triggers intense emotions in China and on the Korean peninsula, where memories linger of Tokyo's brutal expansionism in the last century.

Beijing has sought to paint the prime minister as an atavistic militarist, bent on resurrecting the warmongering of imperialist Japan.

A spokeswoman for China's foreign ministry said on Thursday it had "full reason to be highly vigilant over Japan's true intention and its future development", due to the Abe administration's "unprecendented moves" on military issues.

However, his position is welcomed in Washington, where there have long been calls for Japan to pull its own weight in a very one-sided security alliance.

US President Barack Obama welcomed the move when he held a summit with Abe in Tokyo last month.

Unease in Japan about China's increasing assertiveness, and specifically its strident claims to disputed islands in the East China Sea, has helped bolster Abe's push to enhance the role of the military.