Japan's national bar association on Wednesday threw its weight behind growing opposition to controversial security bills that could pave the way for Japanese troops to engage in combat for the first time since the end of World War II.

Hundreds of demonstrators, including some members of the 36,000-strong Japan Federation of Bar Associations (JFBA), took part in a Tokyo rally with academics and citizens groups calling for the government to scrap the legislation -- in the final stages of working its way through parliament.

Under the proposed new rules, pacifist Japan's Self-Defence Forces would have the option of going into battle to protect allies even if there was no direct threat to Japan or its people.

"The constitution couldn't work as a constitution" if the bills took effect, JFBA chairman Susumu Murakoshi told a news conference ahead of the rain-soaked demonstration.

Murakoshi called the swelling number of lawyers and academics opposed to the bills "unprecedented". Lawyers typically avoid taking part in large scale rallies.

The proposed legislation passed through the powerful lower house of parliament last month and Prime Minister Shinzo Abe's ruling coalition aims to win final approval in the upper house by the end of September.

Abe and his supporters say the changes are necessary for Japan to deal with the world around it, but the push is deeply unpopular among the general public.

Many legal scholars have said the changes are unconstitutional and critics say they will drag Japan into American wars in far-flung parts of the globe.

A constitution imposed by a post-war US occupation force barred Japan's military from combat except in self-defence.