Several thousand people demonstrated to demand that Prime Minister Shinzo Abe's Cabinet scrap its plan, intended to allow the Japanese military to help defend other nations. It would be one of the biggest changes in Japan's security policy since World War II.
The Cabinet is expected to announce the decision tomorrow. The constitution renounces war and has been interpreted as allowing the use of arms only for Japan's own self-defense, and critics say the change undermines the charter.
"Protect the constitution!" they shouted. "Stop war. Stop Abe. Abe quit right now!"
Abe says the revision is needed because of China's military expansion and missile and nuclear threats from North Korea.
Yoshifu Arita, an opposition Democratic Party of Japan lawmaker, joined the rally and said Japan is at a major crossroad. He compared a Cabinet decision to a coup attempt and said it must be stopped.