Japan has launched new security laws that allow its military to engage in conflicts abroad for reasons other than self-defence, a move critics see as a major deviation from the country's pacifist constitution.

For the first time since World War II, Japan will be able to participate more in international peacekeeping compared with its previous, mostly humanitarian, missions.

The legislation, which took effect on Tuesday after being passed last year despite nationwide protests, have been backed by Prime Minister Shinzo Abe and Defence Minister Gen Nakatani, who said it was needed to improve international co-operation of armed forces.

The use of force by the Japanese military has been limited to self-defence since World War II ended in 1945.

On Monday, more than 100 demonstrators gathered in front of the premier's residence, disappointed that the government had ignored their protests against the expansion of military powers and vowed to continue their opposition.

Many young Japanese also rallied across the country on Sunday to oppose the bills.

Osamu Watanabe, honorary professor at Hitotsubashi University, told the Associated Press news agency that that the new laws violated Article IX of the constitution.

"It is the first item that says that Japan will not start a war," Watanabe said.

"The second item says that Japan will not maintain military forces or a fighting army and it will not resort to force as a means of settling international disputes. This is what Article IX says to restrain state power."

Abe's cabinet has denied it is a violation of the constitution, which it said must be adapted to the challenging security situation in the current age.

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The security legislation came against the backdrop of rising regional tensions over territory in the South China Sea, a massive area that includes archipelago where China has a growing military presence.

Watanabe also warned against the legislation leading to conflict in the region.

"For peace in Asia, what Japan can do is avoid having those kind of troops that fought against some Asian countries and dragged Asia into hell," he said.

About 100 Japanese lawyers have set up a lawsuit association and will sue in local courts on new unconstitutional security bills in April.