There were chaotic scenes inside Japan's normally orderly parliament Wednesday as opposition lawmakers thronged a committee room in an unsuccessful bid to block Prime Minister Shinzo Abe's controversial security bills.

Dozens of politicians held signs protesting against what they said was the "forced" passage of legislation that will bolster the role of Japan's military, in a way they say is anathema to the country's pacifist constitution.

The floor of the committee room filled moments after chairman Yasukazu Hamada, a member of Abe's conservative Liberal Democratic Party (LDP), called a vote.

Lawmakers chanted "nay, nay, nay" and held posters saying "No to Abe politics", and "No to a forced decision", as their LDP colleagues pressed on with the vote, which they won comfortably.

"This will drastically change our defence policies. It's also likely they are unconstitutional," said Katsuya Okada, head of main opposition Democratic Party of Japan (DPJ).

"I strongly protest that these bills were forced through the committee," he said.

The bills, which are expected to go to a vote of the powerful -- and LDP-dominated -- lower chamber on Thursday, are something of a pet project for Abe, despite widespread public disquiet over what many Japanese say is an affront to 70 years of pacifism.

Hundreds of people protested outside parliament Wednesday, with numbers expected to swell throughout the evening.

On Tuesday, around 20,000 people rallied against the changes.

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 pacifist constitution imposed by the United States after World War II, Abe opted instead to re-interpret it for the purpose of his bills.

- Political cost -

Chief among the changes that the legislation will enable is the option for the military to go into battle to protect allies -- so called "collective defence" -- even if there is no direct threat to Japan or its people, something successive governments have ruled out.

If, as expected, the lower chamber passes the bills on Thursday, they will go to the upper house.

Abe has been forced to extend the parliamentary session months beyond its normal finish in a bid to get the legislation through both chambers.

But there are growing signs that his determination to push the unpopular bills is exacting a political cost.

Abe's support rate has fallen to 39 percent, lower than the 42 percent disapproval rating, according to the latest poll by the leading Asahi newspaper.

The shift in military policy is supported by just 26 percent of those polled, while 56 percent expressed opposition.

While there is little chance of Abe stepping down, the premier's declining popularity echoes that of his grandfather Nobusuke Kishi, whose support for a security treaty with the US helped bring about his resignation as prime minister in 1960.

"Unfortunately, the Japanese people still don't have a substantial understanding" of the bills, the prime minister told the panel on Wednesday.

"I will work harder so public understanding would deepen further."

Japanese politics abounds with the notion that those who disagree with a position do not understand it properly, and must have it explained to them more carefully.