Japanese Prime Minister Shinzo Abe said that new security laws that took effect Tuesday will strengthen Tokyo's ability to defend itself amid increasing threats as opponents took to the streets to say they risk hurling the country into war.

Legislators in September passed the bills into law, a shift that could see Japanese troops fight abroad for the first time since the end of World War II.

The approval was a milestone in Japan, where a constitution imposed by the United States after Japan's defeat renounces its sovereign right to wage war.

That clause, known as Article 9, is unchanged, but staunch nationalist Abe wants to revise the constitution to throw off what he and many conservatives see as outdated foreign-imposed constraints that hinder Japan from playing a more robust role in regional and global affairs.

"The security environment surrounding our country is becoming more severe," Abe told a nationally televised news conference, citing threats including from nearby North Korea, which this year has carried out fresh nuclear and ballistic rocket launches.

"No country in the world can protect itself alone," he added.

As Abe spoke at his official office, an AFP journalist estimated that about 2,000 people rallied outside parliament nearby in a show of opposition to the laws that opponents fear could fundamentally reshape the proudly pacifist nation.

Fumiko Yamaguchi, who attended the demonstration, said she survived devastating US air raids on Tokyo as a child even as an aunt was killed.

"I don't want Japan to be engaged in any war," she told AFP. "I don't want my children and our grandchildren to go to war."

Abe says that the new laws are part of a normalisation of Japan's military policy, which had been restricted to self-defence and aid missions by the constitution.

The changes, which would allow Japanese troops to fight in defence of allies, drew tens of thousands of protesters onto the streets outside parliament or the prime minister's office in the runup to their passage in September.

Abe and his backers say the laws are necessary because of threats from an increasingly belligerent China and unstable North Korea.

Opponents argue they go against both the constitution and the national psyche, and could see Japan dragged into far-flung US wars.

Washington has backed the changes, but regional rivals China and South Korea have expressed concern at any expansion of Japanese military scope.

Abe also said that the alliance with the US remains strong and will not change after the Us presidential election in November.

"No matter who will be the next president, the Japan-US alliance is the cornerstone of Japan's diplomacy," he said.

"For the sake of the peace and prosperity of Asia, the Pacific and the world, close cooperation with the US won't change."