A growing number of people, including university students and young parents, are joining protests against the controversial bills as Prime Minister Shinzo Abe's ruling party gears up to pass them before the current session ends late next month.
Holding placards reading "No war," "Peace not war" and "Stop the security bills", chanting demonstrators filled the street in front of the Diet (parliament).
A huge banner reading "Abe should step down", adorned with black and white ballons, was carried through the crowd.
Organisers said about 120,000 people took part in the rally in Tokyo, and similar demonstrations were held across Japan. A police estimate was not immediately available.
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Relatively small street demonstrations are frequent in the capital. But on Thursday a group of Tokyo university students staged a rare hunger strike outside parliament to protest at the legislation.
They said they would continue as long as possible.
On Wednesday the national bar association took part in a Tokyo protest rally with academics and citizen groups.
Abe and his supporters say the bills are necessary for Japan to deal with the changed security environment.
But opponents say they will drag Japan into distant American wars, and many legal scholars have said the changes are unconstitutional.
The legislation is deeply unpopular among the general public and public support for Abe's government is declining.
A constitution imposed by a post-war US occupation force barred Japan's military from combat except in self-defence.