Legislation that would allow Japan's military to fight in defence of allies is unconstitutional, legal experts said Monday, as opposition to Prime Minister Shinzo Abe's pet project grows.

Two of Japan's foremost constitutional experts told journalists that the bills, which are currently being debated in parliament, must be withdrawn because they risked undermining the country's avowed pacifism.

"The government should retract the bills, because the core element of the bills -- allowing the use of collective self-defence -- is manifestly unconstitutional," Yasuo Hasebe, professor of constitutional law at Waseda University, told reporters.

"It is quite likely that the bills could bring about the entanglement of (Japan's armed forces) in unconstitutional foreign military activities."

Abe, a robust nationalist, has pushed for what he calls a normalisation of Japan's military posture. He has sought to loosen restrictions that have bound the so-called Self-Defense Forces to a narrowly defensive role for decades.

But unable to muster the public support to amend the constitution imposed by the United States after World War II, Abe opted instead to re-interpret it, and proposed legislation that allows the military greater scope to act.

Chief amongst the changes is the option for it to go into battle even if there is no direct threat to Japan or its people, something successive governments have ruled out.

Washington, which for 70 years has been the guarantor of Japan's security, has welcomed the move, which to many foreign eyes seems relatively uncontroversial.

However, it has proved deeply unpopular among academics and Japan's public, who are deeply wedded to the commitment to pacifism.

Setsu Kobayashi, professor emeritus of Keio University, said passing laws that violated the letter of the constitution was a slippery slope.

"What's scary is that should the political practice of violating the constitution go unchallenged, this country could become something similar to North Korea," he said.

"If we follow Mr Abe's instructions, Japan's Self-Defense Forces would become second-class US troops and as a result Japan would get hurt," he said.

Hasebe and Kobayashi testified to parliament this month that the bills are unconstitutional.

The Monday comments came a day after thousands of Japanese rallied in protest against the plans, accusing Abe of trying to move the country away from pacifism.

The legislation, which would overhaul 10 security-related laws and create a new one, includes removing geographical constraints on logistical support for friendly forces in "situations that would significantly affect Japan's security".

It also says Japan can defend allies "in situations where there is a clear risk that Japan's existence is threatened and its people's rights... are compromised through an attack on a country which has a close relationship with Japan".