Japan’s prime minister, Sanae Takaichi, has called for “advanced discussions” on revising the pacifist constitution, as large demonstrations were held nationwide to oppose any changes to the country’s supreme law.

Speaking during an official visit to Vietnam, Takaichi said the constitution, which was written by US occupation forces after the second world war, “should periodically be updated to reflect the demands of the times”.

Takaichi and others on the conservative wing of the ruling Liberal Democratic party have long called for change, saying the current document restricts Japan’s ability to respond to growing security threats from North Korea and China.

Revisionists have set their sights on article 9 – the co-called “pacifist” clause – which forbids Japan from threatening or using military force to settle international disputes.

While controversial legislation passed a decade ago theoretically allows Japan to exercise collective self-defence – or coming to the aid of an ally under attack – Takaichi has turned reform into a focal point of her administration since becoming prime minister last autumn.

Any revisions would need to secure a two-thirds majority in both houses of Japan’s national diet – or parliament – and a simple majority in a national referendum.

Recent opinion polls reveal deep divisions among the public, from broad support for minor revisions, such as recognising the legal status of the self-defence forces, to opposition to fundamental changes to Japan’s postwar pacifism.

In a poll published at the weekend by the conservative Yomiuri Shimbun newspaper, 57% of respondents were in favour of revision, while a survey by the liberal Asahi Shimbun put support for reform at 47%.

The constitutional constraints placed on Japan’s military were highlighted in March, when Takaichi cited article 9 when she turned down a request – reportedly with reluctance – by Donald Trump to send the maritime self-defence forces to the strait of Hormuz.

On Sunday – constitutional memorial day – an estimated 50,000 people gathered at a park in Tokyo in support of the document, whose wording has remained unchanged since it went into effect on 3 May 1947.

Protesters holding anti-war placards said article 9 had succeeded in keeping Japan out of ill-advised US-wars, including Iran.

“Under Takaichi, Japan is following America like a dog follows its owner,” said Hiroko Maekawa, a councillor for a Tokyo ward. “The LDP wants to turn the self-defence forces into a traditional military, because they know the constitution, as it is, prevents them from doing that.”

Another local councillor, Megumi Koike, described Japan’s constitution as “a national treasure and a treasure to the world”.

“Takaichi thinks that most Japanese people want to change the constitution because they believe there is a threat from China and North Korea, but that’s just not true,” she said. “We should be spending money on healthcare, education and jobs, not on more weapons.”

Demonstrations were held in dozens of other towns and cities on Sunday – a public holiday – attended by people old enough to recall how the postwar constitution had brought peace and stability to a country ravaged by conflict.

“I want to cherish the constitution like I do my own child, and pass it on to the next generation,” Haruka Watanabe, an 87-year-old protester in Osaka, told the Kyodo news agency.

As she prepared to travel to Australia to discuss energy security, critical minerals and defence with the prime minister, Anthony Albanese, Takaichi said the time for debate was almost over. “We mustn’t have discussion just for discussion’s sake,” she said in Hanoi, according to Kyodo.

“To retain the trust placed in them by the people, politicians must discuss the issue and make a decision.”

Although Trump has criticised Japan for not sending troops to the Middle East, the US embassy in Tokyo posted a message that could be interpreted as support for constitution, which went into effect during the US occupation led by Gen Douglas MacArthur, the supreme commander of the Allied Powers.

The document, the embassy said on its official X account, had upheld “popular sovereignty, respect for fundamental human rights, and pacifism”. It added: “This constitution, highly praised by Gen MacArthur in his memoirs, has continued to serve as the foundation of Japanese society for 79 years since its enactment, without ever having been amended.”

Sunday’s protest in Tokyo was the latest in a wave of demonstrations that are attracting people in greater numbers each time. An estimated 3,600 people demonstrated outside parliament in late February, swelling to 36,000 later that month.