Legislation for a dramatic change in Japan's defence policy that could allow troops to fight abroad for the first time since World War Two was approved by a lower house panel on Wednesday, sparking large protests from ordinary voters against the change.
Prime Minister Shinzo Abe says a bolder security stance, welcomed by key ally Washington, is vital to meet new challenges, such as those from a rising China.
Opponents say the revisions violate the post-war constitution's pacifist Article 9 and could entangle Japan in U.S.-led conflicts around the globe.
Ruling coalition lawmakers approved the bills after a raucous debate and a scrum, with opposition MPs trying to block the vote, shouting and brandishing signs that read "Abe politics is unforgivable" and "Against ramming bills through".
The bills are expected to be approved this week by the full lower house, where the ruling bloc has an overwhelming majority, before going to the upper chamber.
Abe now faces his biggest challenge since taking office in December 2012, vowing to revive Japan's stale economy and bolster its defence.
His ratings have slipped over voter concerns about the plan to drop a ban on collective self-defence, or fighting to defend a friendly country under attack, besides doubts on other signature policies.
Crowds of protesters - organisers said 60,000 - gathered near Abe's office on Wednesday evening, carrying banners and chanting "Scrap the war bills", "Protect Article 9" and "Abe, quit." Demonstrations have been growing and more are planned.
Kyodo news agency reported there were protests in other parts of the country.
Abe's disapproval rate rose five points to 42 percent in an Asahi newspaper poll released on Monday, versus a support rate of 39 percent. Fifty-six percent opposed the bills.
"Political veterans are starting to predict that, like his grandfather (Nobusuke) Kishi, he may achieve his goal but have to resign," said independent political analyst Minoru Morita.
Kishi, a wartime cabinet minister who was premier from 1957 to 1960, resigned 55 years ago to the day, on July 15, 1960, because of a public furore after pushing a revised U.S.-Japan security pact through parliament.
Other analysts say Abe's support will suffer, but he will probably survive, given the weakness of rivals inside and outside his party.
Abe has other headaches.
Kyushu Electric Power is expected to next month reboot a nuclear reactor off-line since the 2011 Fukushima disaster, the first such move in nearly two years, despite public misgivings.
Anger is also mounting over ballooning costs for a new stadium for the 2020 summer Olympics, and a clash between Abe's administration and the governor of Okinawa over a U.S. Marine air base could peak in August.
That is also when Abe will issue a potentially controversial statement for the 70th anniversary of the end of World War Two.
Still, Abe could survive to be elected to another three-year term as head of his Liberal Democratic Party in September.
"People think there is something strange about the Abe government, but what is supporting him is the economy," said political commentator Atsuo Ito.