Japan is expected to pass controversial security bills on Friday that critics say could herald the biggest shift in its defence policy for half a century, despite public anger that has seen tens of thousands protest.

The bills are expected to be passed in the upper house controlled by prime minister Shinzo Abe’s ruling coalition after days of fraught debates that at times descended into scuffles, tears and tantrums.

Opposition lawmakers tried every delaying tactic at their disposal, even resorting to physically blocking a vote in a special committee, but it now looks like all of their options have been exhausted.

The controversial laws have seen tens of thousands take to the streets in almost daily rallies for the past few weeks, in a show of public anger on a scale rarely seen in Japan.

Opponents argue the new laws – which would allow the tightly restricted military to intervene overseas to defend its allies – violate Japan’s pacifist constitution and could see the country dragged into American wars in far-flung parts of the globe.

But despite months of fierce opposition, Abe now looks set to enact what critics say could be the biggest shift in Japan’s defence policy since when his grandfather was in power 55 years ago.

“Japan is facing a turning point of its security policy,” said Tomoaki Iwai, professor of politics at Nihon University in Tokyo.

Nationalist Abe wants what he calls a normalisation of Japan’s military posture, which has been constrained from anything but self-defence and aid missions by a pacifist constitution imposed by the US after the second world war.

Unable to muster support to amend clauses enshrining pacifism, Abe opted instead to reinterpret the document for the purpose of his bills, ignoring warnings from scholars and lawyers that they are unconstitutional.

The changes reinterpret the constitution to allow Japan’s military to fight to protect its allies, which Abe argues is necessary because of threats from an increasingly belligerent China and unstable North Korea.

“The security situation surrounding our country has changed much more than we imagined,” the premier told parliament on Monday.

“The legislation is essential in order to protect our people’s lives and peaceful livelihood, and it is necessary to put the legislation in place as soon as possible,” he added.

Still, there are growing signs the campaign has taken a political toll – opinion polls show the vast majority of the public is against the bills, and Abe’s once sky-high approval rating is dropping.

Protesters, including a Nobel Prize winner, popular musicians and other prominent figures, fear the changes could fundamentally alter Japan’s character as a pacifist nation.

“Japanese are often seen as modest and humble, but it’s not the case this time,” said Ryoko Ikeda, a 36-year-old mother at one of the daily rallies against the bills held near parliament for the past weeks.

“It is our children and future generations who pay the price.”

Keiko Nagao, in her 40s, added: “A pacifist image is Japan’s treasure and if we lose it, it will be a big loss for our country.”

Security experts said the bills would also force a re-evaluation of Japan’s place on the world stage.

“The bills are a psychological message to the world that an era in which Japan should not be involved in conflicts because of its exclusively defence-oriented policy is over,” said Hideshi Takesada, a professor at Takushoku University in Tokyo.