Hundreds of people holding placards saying "No war" and "Don't ram through the legislation" faced a line of police outside a hotel in Yokohama, a city south of Tokyo, where lawmakers gathered for a public hearing on the bills.
Demonstrators later began blocking the roads, chanting anti-war slogans and trying to stop lawmakers from driving away after the debate.
The protests are the latest show of public anger at Prime Minister Shinzo Abe's plans to expand the role of Japan's military on the world stage.
Opposition parties are planning actions to try block the legislation, including censure motions to try to embarrass the government into submission, but these are not expected to stop the bill being passed.
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Under the proposed new laws Japan's army, known as the Self-Defence Forces, would be able to go into battle to protect allies even if there was no direct threat to the country or its people.
Many legal scholars have said the changes are unconstitutional, and critics worry they would drag Japan into American wars in far-flung parts of the globe.
Abe and his supporters say the bills are necessary to deal with a changing security environment marked by an increasingly assertive China and unpredictable North Korea.