By Linda Sieg

TOKYO (Reuters) - Laws loosening the limits of Japan's pacifist constitution on its military took effect on Tuesday as surveys showed the public remained divided over a change that allows Japanese troops to fight overseas for the first time since World War Two.

Prime Minister Shinzo Abe has said the security legislation, the biggest change in Japan's defence policy since the creation of its military in 1954, is vital to meet new challenges including a rising China.

Critics say the changes, which triggered mass demonstrations ahead of their enactment last September, violate the pacifist constitution and increase the risk of involvement in foreign wars. Opposition parties plan to campaign for the laws' repeal in an upper house election in July.

The legislation "is vital to prevent wars and protect the people's lives and livelihoods amid an increasingly severe security environment surrounding our country," Chief Cabinet Secretary Yoshihide Suga told a news conference.

"The government will first preserve the peace through diplomacy and there is no change at all in our policy of proactive diplomacy for that purpose," he added.

Japan's ally the United States has welcomed the changes, which allow the military to fight in aid of friendly countries that come under attack if Japan's security is also threatened.

But China, where bitter memories of Tokyo's wartime aggression run deep, has repeatedly expressed concern about the legislation, based on a controversial re-interpretation of the pacifist constitution.

The main opposition Democratic Party and other opposition groups are raising the issue ahead of the upper house election amid speculation Abe may also call a snap poll for the powerful lower chamber. How much traction the issue is unclear.

A voter survey by the Yomiuri newspaper published on Tuesday showed 47 percent did not approve of the changes against 38 percent who did. That compared to 58 percent who opposed the legislation last September versus 31 percent who approved.

However, in a separate survey by the Nikkei business daily, only 35 percent said the legislation should be repealed, while 43 percent said it should remain in place.

(Additional reporting by Kaori Kaneko; Editing by Simon Cameron-Moore)